nat.bend source
nat.bend on the hub · documented module
# bend-mathlib/nat.bend: Nat arithmetic (add, mul, sub, min, max, pow) and order (le, lt, ge, gt).import Base# Zero is a right identity for addition: x + 0 = x.law add_zero: for x: Nat {Nat.add(x, 0n) == x : Nat}def add_zero(x): match x: case 0n: {==} case 1n+p: %add_zero(p) : {1n+Nat.add(p, 0n) == 1n+_ : Nat} {==}# Zero is a left identity for addition: 0 + x = x.law zero_add: for -x: Nat {Nat.add(0n, x) == x : Nat}def zero_add(x): {==}# Adding a successor on the right: n + (m + 1) = (n + m) + 1.law add_succ: for n: Nat for -m: Nat {Nat.add(n, 1n+m) == 1n+Nat.add(n, m) : Nat}def add_succ(n, m): match n: case 0n: {==} case 1n+p: %add_succ(p, m) : {1n+Nat.add(p, 1n+m) == 1n+_ : Nat} {==}# Adding a successor on the left: (n + 1) + m = (n + m) + 1.law succ_add: for -n: Nat for -m: Nat {Nat.add(1n+n, m) == 1n+Nat.add(n, m) : Nat}def succ_add(n, m): {==}# Addition is commutative: n + m = m + n.law add_comm: for n: Nat for m: Nat {Nat.add(n, m) == Nat.add(m, n) : Nat}def add_comm(n, m): match n m: case 0n 0n: {==} case 0n 1n+q: %add_zero(q) : {1n+_ == 1n+Nat.add(q, 0n) : Nat} {==} case 1n+p 0n: %add_zero(p) : {1n+Nat.add(p, 0n) == 1n+_ : Nat} {==} case 1n++p 1n++q: %Equal.sym(Nat, Nat.add(p, 1n+q), 1n+Nat.add(p, q), add_succ(p, q)) : {1n+_ == 1n+Nat.add(q, 1n+p) : Nat} %Equal.sym(Nat, Nat.add(q, 1n+p), 1n+Nat.add(q, p), add_succ(q, p)) : {2n+Nat.add(p, q) == 1n+_ : Nat} %add_comm(p, q) : {2n+Nat.add(p, q) == 2n+_ : Nat} {==}# Addition is associative: (a + b) + c = a + (b + c).law add_assoc: for a: Nat for -b: Nat for -c: Nat {Nat.add(Nat.add(a, b), c) == Nat.add(a, Nat.add(b, c)) : Nat}def add_assoc(a, b, c): match a: case 0n: {==} case 1n+p: %add_assoc(p, b, c) : {1n+Nat.add(Nat.add(p, b), c) == 1n+_ : Nat} {==}# Left commutativity of addition: a + (b + c) = b + (a + c).law add_left_comm: for a: Nat for b: Nat for -c: Nat {Nat.add(a, Nat.add(b, c)) == Nat.add(b, Nat.add(a, c)) : Nat}def add_left_comm(a, b, c): match a: case 0n: {==} case 1n+p: +b = b %Equal.sym(Nat, Nat.add(b, 1n+Nat.add(p, c)), 1n+Nat.add(b, Nat.add(p, c)), add_succ(b, Nat.add(p, c))) : {1n+Nat.add(p, Nat.add(b, c)) == _ : Nat} %add_left_comm(p, b, c) : {1n+Nat.add(p, Nat.add(b, c)) == 1n+_ : Nat} {==}# Right commutativity of addition: (a + b) + c = (a + c) + b.law add_right_comm: for a: Nat for b: Nat for c: Nat {Nat.add(Nat.add(a, b), c) == Nat.add(Nat.add(a, c), b) : Nat}def add_right_comm(a, b, c): match a: case 0n: add_comm(b, c) case 1n+p: %add_right_comm(p, b, c) : {1n+Nat.add(Nat.add(p, b), c) == 1n+_ : Nat} {==}# Four-way regrouping of a sum: (a + b) + (c + d) = (a + c) + (b + d).law add_add_add_comm: for a: Nat for b: Nat for c: Nat for -d: Nat {Nat.add(Nat.add(a, b), Nat.add(c, d)) == Nat.add(Nat.add(a, c), Nat.add(b, d)) : Nat}def add_add_add_comm(a, b, c, d): match a: case 0n: add_left_comm(b, c, d) case 1n+p: %add_add_add_comm(p, b, c, d) : {1n+Nat.add(Nat.add(p, b), Nat.add(c, d)) == 1n+_ : Nat} {==}def internal_pred(n: Nat) -> Nat: match n: case 0n: 0n case 1n+p: p# The successor function is injective: a + 1 = b + 1 implies a = b.law succ_inj: for -a: Nat for -b: Nat for e: {1n+a == 1n+b : Nat} {a == b : Nat}def succ_inj(a, b, e): %e : {a == internal_pred(_) : Nat} {==}def internal_zero_ne_succ(-n: Nat, e: {0n == 1n+n : Nat}) -> Empty: %e : Bool.pick(Type, Nat.is_eq(_, 0n), Unit, Empty) Unit{}# Zero is not a successor.law zero_ne_succ: for -n: Nat {0n != 1n+n : Nat}def zero_ne_succ(n): e => internal_zero_ne_succ(n, e)def internal_succ_ne_zero(-n: Nat, e: {1n+n == 0n : Nat}) -> Empty: %e : Bool.pick(Type, Nat.is_eq(_, 0n), Empty, Unit) Unit{}# A successor is not zero.law succ_ne_zero: for -n: Nat {1n+n != 0n : Nat}def succ_ne_zero(n): e => internal_succ_ne_zero(n, e)# Addition cancels on the left: a + b = a + c implies b = c.law add_left_cancel: for a: Nat for -b: Nat for -c: Nat for e: {Nat.add(a, b) == Nat.add(a, c) : Nat} {b == c : Nat}def add_left_cancel(a, b, c, e): match a: case 0n: e case 1n+p: add_left_cancel(p, b, c, succ_inj(Nat.add(p, b), Nat.add(p, c), e))# Addition cancels on the right: a + b = c + b implies a = c.law add_right_cancel: for a: Nat for b: Nat for c: Nat for e: {Nat.add(a, b) == Nat.add(c, b) : Nat} {a == c : Nat}def add_right_cancel(a, b, c, e): +b = b add_left_cancel(b, a, c, Equal.trans(Nat, Nat.add(b, a), Nat.add(a, b), Nat.add(b, c), add_comm(b, a), Equal.trans(Nat, Nat.add(a, b), Nat.add(c, b), Nat.add(b, c), e, add_comm(c, b))))# Zero absorbs multiplication on the right: x * 0 = 0.law mul_zero: for x: Nat {Nat.mul(x, 0n) == 0n : Nat}def mul_zero(x): match x: case 0n: {==} case 1n+p: mul_zero(p)# Zero absorbs multiplication on the left: 0 * x = 0.law zero_mul: for -x: Nat {Nat.mul(0n, x) == 0n : Nat}def zero_mul(x): {==}# One is a right identity for multiplication: x * 1 = x.law mul_one: for x: Nat {Nat.mul(x, 1n) == x : Nat}def mul_one(x): match x: case 0n: {==} case 1n+p: %mul_one(p) : {1n+Nat.mul(p, 1n) == 1n+_ : Nat} {==}# One is a left identity for multiplication: 1 * x = x.law one_mul: for x: Nat {Nat.mul(1n, x) == x : Nat}def one_mul(x): add_zero(x)# Multiplying by a successor on the right: n * (m + 1) = n * m + n.law mul_succ: for n: Nat for m: Nat {Nat.mul(n, 1n+m) == Nat.add(Nat.mul(n, m), n) : Nat}def mul_succ(n, m): match n: case 0n: {==} case 1n++p: +m = m %Equal.sym(Nat, Nat.add(Nat.add(m, Nat.mul(p, m)), 1n+p), 1n+Nat.add(Nat.add(m, Nat.mul(p, m)), p), add_succ(Nat.add(m, Nat.mul(p, m)), p)) : {1n+Nat.add(m, Nat.mul(p, 1n+m)) == _ : Nat} %Equal.sym(Nat, Nat.add(Nat.add(m, Nat.mul(p, m)), p), Nat.add(m, Nat.add(Nat.mul(p, m), p)), add_assoc(m, Nat.mul(p, m), p)) : {1n+Nat.add(m, Nat.mul(p, 1n+m)) == 1n+_ : Nat} %mul_succ(p, m) : {1n+Nat.add(m, Nat.mul(p, 1n+m)) == 1n+Nat.add(m, _) : Nat} {==}# Multiplying by a successor on the left: (n + 1) * m = n * m + m.law succ_mul: for n: Nat for m: Nat {Nat.mul(1n+n, m) == Nat.add(Nat.mul(n, m), m) : Nat}def succ_mul(n, m): +m = m add_comm(m, Nat.mul(n, m))# Multiplication is commutative: n * m = m * n.law mul_comm: for n: Nat for m: Nat {Nat.mul(n, m) == Nat.mul(m, n) : Nat}def mul_comm(n, m): match n: case 0n: %mul_zero(m) : {_ == Nat.mul(m, 0n) : Nat} {==} case 1n++p: +m = m %Equal.sym(Nat, Nat.mul(m, 1n+p), Nat.add(Nat.mul(m, p), m), mul_succ(m, p)) : {Nat.add(m, Nat.mul(p, m)) == _ : Nat} %mul_comm(p, m) : {Nat.add(m, Nat.mul(p, m)) == Nat.add(_, m) : Nat} add_comm(m, Nat.mul(p, m))# Multiplication distributes over addition on the right: (a + b) * c = a * c + b * c.law add_mul: for a: Nat for -b: Nat for c: Nat {Nat.mul(Nat.add(a, b), c) == Nat.add(Nat.mul(a, c), Nat.mul(b, c)) : Nat}def add_mul(a, b, c): match a: case 0n: {==} case 1n+p: +c = c %Equal.sym(Nat, Nat.add(Nat.add(c, Nat.mul(p, c)), Nat.mul(b, c)), Nat.add(c, Nat.add(Nat.mul(p, c), Nat.mul(b, c))), add_assoc(c, Nat.mul(p, c), Nat.mul(b, c))) : {Nat.add(c, Nat.mul(Nat.add(p, b), c)) == _ : Nat} %add_mul(p, b, c) : {Nat.add(c, Nat.mul(Nat.add(p, b), c)) == Nat.add(c, _) : Nat} {==}# Multiplication distributes over addition on the left: a * (b + c) = a * b + a * c.law mul_add: for a: Nat for b: Nat for c: Nat {Nat.mul(a, Nat.add(b, c)) == Nat.add(Nat.mul(a, b), Nat.mul(a, c)) : Nat}def mul_add(a, b, c): match a: case 0n: {==} case 1n++p: +b = b +c = c %Equal.sym(Nat, Nat.mul(p, Nat.add(b, c)), Nat.add(Nat.mul(p, b), Nat.mul(p, c)), mul_add(p, b, c)) : {Nat.add(Nat.add(b, c), _) == Nat.add(Nat.add(b, Nat.mul(p, b)), Nat.add(c, Nat.mul(p, c))) : Nat} add_add_add_comm(b, c, Nat.mul(p, b), Nat.mul(p, c))# Multiplication is associative: (a * b) * c = a * (b * c).law mul_assoc: for a: Nat for b: Nat for c: Nat {Nat.mul(Nat.mul(a, b), c) == Nat.mul(a, Nat.mul(b, c)) : Nat}def mul_assoc(a, b, c): match a: case 0n: {==} case 1n+p: +b = b +c = c %mul_assoc(p, b, c) : {Nat.mul(Nat.add(b, Nat.mul(p, b)), c) == Nat.add(Nat.mul(b, c), _) : Nat} add_mul(b, Nat.mul(p, b), c)# The order a <= b on naturals, as a reusable proposition.def le(a: Nat, b: Nat) -> Data: {Nat.is_le(a, b) == True{} : Bool}# The strict order a < b on naturals, as a reusable proposition.def lt(a: Nat, b: Nat) -> Data: {Nat.is_lt(a, b) == True{} : Bool}# The order a >= b on naturals, as a reusable proposition.def ge(a: Nat, b: Nat) -> Data: {Nat.is_ge(a, b) == True{} : Bool}# The strict order a > b on naturals, as a reusable proposition.def gt(a: Nat, b: Nat) -> Data: {Nat.is_gt(a, b) == True{} : Bool}def internal_false_ne_true(e: {False{} == True{} : Bool}) -> Empty: %e : Bool.pick(Type, _, Empty, Unit) Unit{}# Every natural is at most itself: a <= a.law le_refl: for a: Nat le(a, a)def le_refl(a): match a: case 0n: {==} case 1n+p: le_refl(p)# Zero is at most every natural: 0 <= b.law zero_le: for b: Nat le(0n, b)def zero_le(b): match b: case 0n: {==} case 1n+p: {==}# Every natural is at most its successor: n <= n + 1.law le_succ: for n: Nat le(n, 1n+n)def le_succ(n): match n: case 0n: {==} case 1n+p: le_succ(p)# Adding on the right never decreases a natural: n <= n + k.law le_add_right: for n: Nat for k: Nat le(n, Nat.add(n, k))def le_add_right(n, k): match n: case 0n: zero_le(k) case 1n+p: le_add_right(p, k)# The order is transitive: a <= b and b <= c imply a <= c.law le_trans: for a: Nat for b: Nat for c: Nat for ab: le(a, b) for bc: le(b, c) le(a, c)def le_trans(a, b, c, ab, bc): match a b c: case 0n _ 0n: {==} case 0n _ 1n+r: {==} case 1n+p 0n _: Empty.absurd(le(1n+p, c), internal_false_ne_true(ab)) case 1n+p 1n+q 0n: Empty.absurd(le(1n+p, 0n), internal_false_ne_true(bc)) case 1n+p 1n+q 1n+r: le_trans(p, q, r, ab, bc)# The order is antisymmetric: a <= b and b <= a imply a = b.law le_antisymm: for a: Nat for b: Nat for ab: le(a, b) for ba: le(b, a) {a == b : Nat}def le_antisymm(a, b, ab, ba): match a b: case 0n 0n: {==} case 0n 1n+q: Empty.absurd({0n == 1n+q : Nat}, internal_false_ne_true(ba)) case 1n+p 0n: Empty.absurd({1n+p == 0n : Nat}, internal_false_ne_true(ab)) case 1n+p 1n+q: %le_antisymm(p, q, ab, ba) : {1n+p == 1n+_ : Nat} {==}# The order is total: a <= b or b <= a.law le_total: for a: Nat for b: Nat Or(le(a, b), le(b, a))def le_total(a, b): match a b: case 0n 0n: Inl{{==}} case 0n 1n+q: Inl{{==}} case 1n+p 0n: Inr{{==}} case 1n+p 1n+q: le_total(p, q)# The order is total, as a reusable sum: a <= b or b <= a.law le_total_d: for a: Nat for b: Nat Either<&2, &2, le(a, b), le(b, a)>def le_total_d(a, b): match a b: case 0n 0n: Inl{{==}} case 0n 1n+q: Inl{{==}} case 1n+p 0n: Inr{{==}} case 1n+p 1n+q: le_total_d(p, q)# No natural is less than itself.law lt_irrefl: for a: Nat lt(a, a) -> Emptydef lt_irrefl(a): match a: case 0n: h => internal_false_ne_true(h) case 1n+p: lt_irrefl(p)# The strict order is transitive: a < b and b < c imply a < c.law lt_trans: for a: Nat for b: Nat for c: Nat for ab: lt(a, b) for bc: lt(b, c) lt(a, c)def lt_trans(a, b, c, ab, bc): match a b c: case 0n 0n _: Empty.absurd(lt(0n, c), internal_false_ne_true(ab)) case 0n 1n+q 0n: Empty.absurd(lt(0n, 0n), internal_false_ne_true(bc)) case 0n 1n+q 1n+r: {==} case 1n+p 0n _: Empty.absurd(lt(1n+p, c), internal_false_ne_true(ab)) case 1n+p 1n+q 0n: Empty.absurd(lt(1n+p, 0n), internal_false_ne_true(bc)) case 1n+p 1n+q 1n+r: lt_trans(p, q, r, ab, bc)# A strict inequality implies the weak one: a < b implies a <= b.law le_of_lt: for a: Nat for b: Nat for h: lt(a, b) le(a, b)def le_of_lt(a, b, h): match a b: case 0n 0n: {==} case 0n 1n+q: {==} case 1n+p 0n: Empty.absurd(le(1n+p, 0n), internal_false_ne_true(h)) case 1n+p 1n+q: le_of_lt(p, q, h)# Flipping a >= b gives b <= a.law le_of_ge: for a: Nat for b: Nat for h: ge(a, b) le(b, a)def le_of_ge(a, b, h): match a b: case 0n 0n: {==} case 0n 1n+q: Empty.absurd(le(1n+q, 0n), internal_false_ne_true(h)) case 1n+p 0n: {==} case 1n+p 1n+q: le_of_ge(p, q, h)# Flipping b <= a gives a >= b.law ge_of_le: for a: Nat for b: Nat for h: le(b, a) ge(a, b)def ge_of_le(a, b, h): match a b: case 0n 0n: {==} case 0n 1n+q: Empty.absurd(ge(0n, 1n+q), internal_false_ne_true(h)) case 1n+p 0n: {==} case 1n+p 1n+q: ge_of_le(p, q, h)# Flipping a > b gives b < a.law lt_of_gt: for a: Nat for b: Nat for h: gt(a, b) lt(b, a)def lt_of_gt(a, b, h): match a b: case 0n 0n: Empty.absurd(lt(0n, 0n), internal_false_ne_true(h)) case 0n 1n+q: Empty.absurd(lt(1n+q, 0n), internal_false_ne_true(h)) case 1n+p 0n: {==} case 1n+p 1n+q: lt_of_gt(p, q, h)# Flipping b < a gives a > b.law gt_of_lt: for a: Nat for b: Nat for h: lt(b, a) gt(a, b)def gt_of_lt(a, b, h): match a b: case 0n 0n: Empty.absurd(gt(0n, 0n), internal_false_ne_true(h)) case 0n 1n+q: Empty.absurd(gt(0n, 1n+q), internal_false_ne_true(h)) case 1n+p 0n: {==} case 1n+p 1n+q: gt_of_lt(p, q, h)# Subtracting zero changes nothing: n - 0 = n.law sub_zero: for n: Nat {Nat.sub(n, 0n) == n : Nat}def sub_zero(n): match n: case 0n: {==} case 1n+p: {==}# Truncated subtraction from zero is zero: 0 - n = 0.law zero_sub: for n: Nat {Nat.sub(0n, n) == 0n : Nat}def zero_sub(n): match n: case 0n: {==} case 1n+p: {==}# A natural minus itself is zero: n - n = 0.law sub_self: for n: Nat {Nat.sub(n, n) == 0n : Nat}def sub_self(n): match n: case 0n: {==} case 1n+p: sub_self(p)# Subtracting successors: (n + 1) - (m + 1) = n - m.law succ_sub_succ: for -n: Nat for -m: Nat {Nat.sub(1n+n, 1n+m) == Nat.sub(n, m) : Nat}def succ_sub_succ(n, m): {==}# Adding then subtracting m cancels: (n + m) - m = n.law add_sub_cancel: for n: Nat for m: Nat {Nat.sub(Nat.add(n, m), m) == n : Nat}def add_sub_cancel(n, m): match m: case 0n: +n = n %Equal.sym(Nat, Nat.add(n, 0n), n, add_zero(n)) : {Nat.sub(_, 0n) == n : Nat} sub_zero(n) case 1n++q: +n = n %Equal.sym(Nat, Nat.add(n, 1n+q), 1n+Nat.add(n, q), add_succ(n, q)) : {Nat.sub(_, 1n+q) == n : Nat} add_sub_cancel(n, q)# Adding then subtracting n cancels: (n + m) - n = m.law add_sub_cancel_left: for n: Nat for m: Nat {Nat.sub(Nat.add(n, m), n) == m : Nat}def add_sub_cancel_left(n, m): match n: case 0n: sub_zero(m) case 1n+p: add_sub_cancel_left(p, m)# If m <= n, subtracting and adding m back gives n: (n - m) + m = n.law sub_add_cancel: for n: Nat for m: Nat for h: le(m, n) {Nat.add(Nat.sub(n, m), m) == n : Nat}def sub_add_cancel(n, m, h): match n m: case 0n 0n: {==} case 0n 1n+q: Empty.absurd({1n+q == 0n : Nat}, internal_false_ne_true(h)) case 1n++p 0n: %add_zero(p) : {1n+Nat.add(p, 0n) == 1n+_ : Nat} {==} case 1n++p 1n++q: %Equal.sym(Nat, Nat.add(Nat.sub(p, q), 1n+q), 1n+Nat.add(Nat.sub(p, q), q), add_succ(Nat.sub(p, q), q)) : {_ == 1n+p : Nat} %Equal.sym(Nat, Nat.add(Nat.sub(p, q), q), p, sub_add_cancel(p, q, h)) : {1n+_ == 1n+p : Nat} {==}# Subtracting twice is subtracting the sum: (n - m) - k = n - (m + k).law sub_sub: for n: Nat for m: Nat for k: Nat {Nat.sub(Nat.sub(n, m), k) == Nat.sub(n, Nat.add(m, k)) : Nat}def sub_sub(n, m, k): match n m: case 0n 0n: {==} case 0n 1n+q: zero_sub(k) case 1n+p 0n: {==} case 1n+p 1n+q: sub_sub(p, q, k)# Truncated subtraction never increases: n - m <= n.law sub_le: for n: Nat for m: Nat le(Nat.sub(n, m), n)def sub_le(n, m): match n m: case 0n 0n: {==} case 0n 1n+q: {==} case 1n+p 0n: le_refl(1n+p) case 1n++p 1n++q: le_trans(Nat.sub(p, q), p, 1n+p, sub_le(p, q), le_succ(p))def internal_div_true_ne_false(h: {True{} == False{} : Bool}) -> Empty: internal_false_ne_true(Equal.sym(Bool, True{}, False{}, h))def internal_div_add_zero_r(a: Nat) -> {a == Nat.add(a, 0n) : Nat}: Equal.sym(Nat, Nat.add(a, 0n), a, add_zero(a))def internal_div_le_refl(a: Nat) -> {True{} == Nat.is_le(a, a) : Bool}: Equal.sym(Bool, Nat.is_le(a, a), True{}, le_refl(a))def internal_div_zero_le(x: Nat) -> {True{} == Nat.is_le(0n, x) : Bool}: Equal.sym(Bool, Nat.is_le(0n, x), True{}, zero_le(x))def internal_div_block_start(-bp: Nat, +r: Nat, h: {bp == Nat.add(r, 0n) : Nat}) -> {bp == r : Nat}: %add_zero(r) : {bp == _ : Nat} hdef internal_div_block_step(-bp: Nat, r: Nat, -mp: Nat, h: {bp == Nat.add(r, 1n+mp) : Nat}) -> {bp == 1n+Nat.add(r, mp) : Nat}: %add_succ(r, mp) : {bp == _ : Nat} hdef internal_div_go_eq(n: Nat, m: Nat, +bp: Nat, +d: Nat, +r: Nat, +h: {bp == Nat.add(r, m) : Nat}) -> {Nat.add(Nat.mul(Pair.fst(Nat, Nat, Nat.divmod.go(n, m, d, r)), 1n+bp), Pair.snd(Nat, Nat, Nat.divmod.go(n, m, d, r))) == Nat.add(n, Nat.add(Nat.mul(d, 1n+bp), r)) : Nat}: match n m: case 0n _: {==} case 1n++np 0n: %add_succ(np, Nat.add(Nat.mul(d, 1n+bp), r)) : {Nat.add(Nat.mul(Pair.fst(Nat, Nat, Nat.divmod.go(np, r, 1n+d, 0n)), 1n+bp), Pair.snd(Nat, Nat, Nat.divmod.go(np, r, 1n+d, 0n))) == _ : Nat} %add_comm(r, Nat.mul(d, 1n+bp)) : {Nat.add(Nat.mul(Pair.fst(Nat, Nat, Nat.divmod.go(np, r, 1n+d, 0n)), 1n+bp), Pair.snd(Nat, Nat, Nat.divmod.go(np, r, 1n+d, 0n))) == Nat.add(np, 1n+_) : Nat} %internal_div_block_start(bp, r, h) : {Nat.add(Nat.mul(Pair.fst(Nat, Nat, Nat.divmod.go(np, r, 1n+d, 0n)), 1n+bp), Pair.snd(Nat, Nat, Nat.divmod.go(np, r, 1n+d, 0n))) == Nat.add(np, 1n+Nat.add(_, Nat.mul(d, 1n+bp))) : Nat} %add_zero(Nat.add(bp, Nat.mul(d, 1n+bp))) : {Nat.add(Nat.mul(Pair.fst(Nat, Nat, Nat.divmod.go(np, r, 1n+d, 0n)), 1n+bp), Pair.snd(Nat, Nat, Nat.divmod.go(np, r, 1n+d, 0n))) == Nat.add(np, 1n+_) : Nat} internal_div_go_eq(np, r, bp, 1n+d, 0n, internal_div_block_start(bp, r, h)) case 1n++np 1n++mp: %add_succ(np, Nat.add(Nat.mul(d, 1n+bp), r)) : {Nat.add(Nat.mul(Pair.fst(Nat, Nat, Nat.divmod.go(np, mp, d, 1n+r)), 1n+bp), Pair.snd(Nat, Nat, Nat.divmod.go(np, mp, d, 1n+r))) == _ : Nat} %add_succ(Nat.mul(d, 1n+bp), r) : {Nat.add(Nat.mul(Pair.fst(Nat, Nat, Nat.divmod.go(np, mp, d, 1n+r)), 1n+bp), Pair.snd(Nat, Nat, Nat.divmod.go(np, mp, d, 1n+r))) == Nat.add(np, _) : Nat} internal_div_go_eq(np, mp, bp, d, 1n+r, internal_div_block_step(bp, r, mp, h))# The division equation, for a positive divisor 1 + b: (a / (1 + b)) * (1 + b) + a % (1 + b) = a.law div_mod_eq: for +a: Nat for +b: Nat {Nat.add(Nat.mul(Nat.div(a, 1n+b), 1n+b), Nat.mod(a, 1n+b)) == a : Nat}def div_mod_eq(a, b): %add_zero(a) : {Nat.add(Nat.mul(Nat.div(a, 1n+b), 1n+b), Nat.mod(a, 1n+b)) == _ : Nat} internal_div_go_eq(a, b, b, 0n, 0n, {==})def internal_div_wit_succ(+ap: Nat, +bq: Nat, w: &k:Nat -> {Nat.add(ap, k) == bq : Nat}) -> &k:Nat -> {Nat.add(1n+ap, k) == 1n+bq : Nat}: match w: case (k, e): (k, Equal.cong(Nat, Nat, x => 1n+x, Nat.add(ap, k), bq, e))def internal_div_le_wit(a: Nat, b: Nat, h: {Nat.is_le(a, b) == True{} : Bool}) -> &k:Nat -> {Nat.add(a, k) == b : Nat}: match a b: case 0n y: (y, {==}) case 1n+ap 0n: Empty.absurd(&k:Nat -> {Nat.add(1n+ap, k) == 0n : Nat}, internal_false_ne_true(h)) case 1n++ap 1n++bq: internal_div_wit_succ(ap, bq, internal_div_le_wit(ap, bq, h))def internal_div_go_small(n: Nat, +k: Nat, +d: Nat, r: Nat) -> {d == Pair.fst(Nat, Nat, Nat.divmod.go(n, Nat.add(n, k), d, r)) : Nat}: match n: case 0n: {==} case 1n+np: internal_div_go_small(np, k, d, 1n+r)def internal_div_zero_of_wit(+a: Nat, +b: Nat, w: &k:Nat -> {Nat.add(a, k) == b : Nat}) -> {Nat.div(a, 1n+b) == 0n : Nat}: match w: case (+k, e): %e : {Nat.div(a, 1n+_) == 0n : Nat} Equal.sym(Nat, 0n, Pair.fst(Nat, Nat, Nat.divmod.go(a, Nat.add(a, k), 0n, 0n)), internal_div_go_small(a, k, 0n, 0n))# A number at most b divides by 1 + b to zero: a <= b implies a / (1 + b) = 0.law div_eq_zero_of_le: for +a: Nat for +b: Nat for h: le(a, b) {Nat.div(a, 1n+b) == 0n : Nat}def div_eq_zero_of_le(a, b, h): internal_div_zero_of_wit(a, b, internal_div_le_wit(a, b, h))def internal_div_le_succ_r(j: Nat, d: Nat, h: {True{} == Nat.is_le(j, d) : Bool}) -> {True{} == Nat.is_le(j, 1n+d) : Bool}: match j d: case 0n _: {==} case 1n+i 0n: Empty.absurd({True{} == Nat.is_le(1n+i, 1n) : Bool}, internal_div_true_ne_false(h)) case 1n+i 1n+e: internal_div_le_succ_r(i, e, h)def internal_div_go_d_ge(+j: Nat, n: Nat, m: Nat, +d: Nat, r: Nat, h: {True{} == Nat.is_le(j, d) : Bool}) -> {True{} == Nat.is_le(j, Pair.fst(Nat, Nat, Nat.divmod.go(n, m, d, r))) : Bool}: match n m: case 0n _: h case 1n+np 0n: internal_div_go_d_ge(j, np, r, 1n+d, 0n, internal_div_le_succ_r(j, d, h)) case 1n+np 1n+mp: internal_div_go_d_ge(j, np, mp, d, 1n+r, h)def internal_div_mono_go(a: Nat, m: Nat, +d: Nat, r: Nat, +k: Nat) -> {True{} == Nat.is_le(Pair.fst(Nat, Nat, Nat.divmod.go(a, m, d, r)), Pair.fst(Nat, Nat, Nat.divmod.go(Nat.add(a, k), m, d, r))) : Bool}: match a m: case 0n _: internal_div_go_d_ge(d, k, m, d, r, internal_div_le_refl(d)) case 1n+ap 0n: internal_div_mono_go(ap, r, 1n+d, 0n, k) case 1n+ap 1n+mp: internal_div_mono_go(ap, mp, d, 1n+r, k)def internal_div_mono_wit(+a: Nat, +c: Nat, b: Nat, w: &k:Nat -> {Nat.add(a, k) == c : Nat}) -> le(Nat.div(a, 1n+b), Nat.div(c, 1n+b)): match w: case (+k, e): %e : le(Nat.div(a, 1n+b), Nat.div(_, 1n+b)) Equal.sym(Bool, True{}, Nat.is_le(Nat.div(a, 1n+b), Nat.div(Nat.add(a, k), 1n+b)), internal_div_mono_go(a, b, 0n, 0n, k))# Division by a positive divisor is monotone: a <= c implies a / (1 + b) <= c / (1 + b).law div_le_div: for +a: Nat for +c: Nat for b: Nat for h: le(a, c) le(Nat.div(a, 1n+b), Nat.div(c, 1n+b))def div_le_div(a, c, b, h): internal_div_mono_wit(a, c, b, internal_div_le_wit(a, c, h))def internal_div_go_shift(n: Nat, m: Nat, +d: Nat, +r: Nat) -> {1n+Pair.fst(Nat, Nat, Nat.divmod.go(n, m, d, r)) == Pair.fst(Nat, Nat, Nat.divmod.go(n, m, 1n+d, r)) : Nat}: match n m: case 0n _: {==} case 1n+np 0n: internal_div_go_shift(np, r, 1n+d, 0n) case 1n+np 1n+mp: internal_div_go_shift(np, mp, d, 1n+r)def internal_div_go_run(m: Nat, +a: Nat, +d: Nat, +r: Nat) -> {Nat.divmod.go(a, Nat.add(m, r), 1n+d, 0n) == Nat.divmod.go(Nat.add(1n+m, a), m, d, r) : Nat & Nat}: match m: case 0n: {==} case 1n++mp: %add_succ(mp, r) : {Nat.divmod.go(a, _, 1n+d, 0n) == Nat.divmod.go(1n+Nat.add(mp, a), mp, d, 1n+r) : Nat & Nat} internal_div_go_run(mp, a, d, 1n+r)# Adding the divisor adds one to the quotient: ((1 + b) + a) / (1 + b) = 1 + a / (1 + b).law add_div_left: for +a: Nat for +b: Nat {Nat.div(Nat.add(1n+b, a), 1n+b) == 1n+Nat.div(a, 1n+b) : Nat}def add_div_left(a, b): %internal_div_go_run(b, a, 0n, 0n) : {Pair.fst(Nat, Nat, _) == 1n+Nat.div(a, 1n+b) : Nat} %internal_div_add_zero_r(b) : {Pair.fst(Nat, Nat, Nat.divmod.go(a, _, 1n, 0n)) == 1n+Nat.div(a, 1n+b) : Nat} Equal.sym(Nat, 1n+Nat.div(a, 1n+b), Pair.fst(Nat, Nat, Nat.divmod.go(a, b, 1n, 0n)), internal_div_go_shift(a, b, 0n, 0n))def internal_div_le_add_cancel(t: Nat, +x: Nat, +y: Nat) -> {Nat.is_le(x, y) == Nat.is_le(Nat.add(t, x), Nat.add(t, y)) : Bool}: match t: case 0n: {==} case 1n+tp: internal_div_le_add_cancel(tp, x, y)def internal_div_gt_add(a: Nat, +y: Nat) -> {False{} == Nat.is_le(1n+Nat.add(a, y), a) : Bool}: match a: case 0n: {==} case 1n+ap: internal_div_gt_add(ap, y)def internal_div_not_succ_le(a: Nat, bp: Nat, h: {False{} == Nat.is_le(1n+bp, a) : Bool}) -> {Nat.is_le(a, bp) == True{} : Bool}: match a bp: case 0n q: Equal.sym(Bool, True{}, Nat.is_le(0n, q), internal_div_zero_le(q)) case 1n+ap 0n: Empty.absurd({Nat.is_le(1n+ap, 0n) == True{} : Bool}, internal_div_true_ne_false(Equal.sym(Bool, False{}, True{}, Equal.trans(Bool, False{}, Nat.is_le(0n, ap), True{}, h, Equal.sym(Bool, True{}, Nat.is_le(0n, ap), internal_div_zero_le(ap)))))) case 1n+ap 1n+q: internal_div_not_succ_le(ap, q, h)def internal_div_le_big(+np: Nat, +a: Nat, +bp: Nat, w: &k:Nat -> {Nat.add(1n+bp, k) == a : Nat}, ih: @x:Nat -> {Nat.is_le(np, Nat.div(x, 1n+bp)) == Nat.is_le(Nat.mul(np, 1n+bp), x) : Bool}) -> {Nat.is_le(1n+np, Nat.div(a, 1n+bp)) == Nat.is_le(Nat.mul(1n+np, 1n+bp), a) : Bool}: match w: case (+k, e): %e : {Nat.is_le(1n+np, Nat.div(_, 1n+bp)) == Nat.is_le(1n+Nat.add(bp, Nat.mul(np, 1n+bp)), _) : Bool} %internal_div_le_add_cancel(bp, Nat.mul(np, 1n+bp), k) : {Nat.is_le(1n+np, Nat.div(1n+Nat.add(bp, k), 1n+bp)) == _ : Bool} %Equal.sym(Nat, Nat.div(Nat.add(1n+bp, k), 1n+bp), 1n+Nat.div(k, 1n+bp), add_div_left(k, bp)) : {Nat.is_le(1n+np, _) == Nat.is_le(Nat.mul(np, 1n+bp), k) : Bool} ih(k)def internal_div_le_small(+np: Nat, +a: Nat, +bp: Nat, w: &k:Nat -> {Nat.add(a, k) == bp : Nat}) -> {Nat.is_le(1n+np, Nat.div(a, 1n+bp)) == Nat.is_le(Nat.mul(1n+np, 1n+bp), a) : Bool}: match w: case (+k, e): %e : {Nat.is_le(1n+np, Nat.div(a, 1n+_)) == Nat.is_le(1n+Nat.add(_, Nat.mul(np, 1n+_)), a) : Bool} %internal_div_go_small(a, k, 0n, 0n) : {Nat.is_le(1n+np, _) == Nat.is_le(1n+Nat.add(Nat.add(a, k), Nat.mul(np, 1n+Nat.add(a, k))), a) : Bool} %Equal.sym(Nat, Nat.add(Nat.add(a, k), Nat.mul(np, 1n+Nat.add(a, k))), Nat.add(a, Nat.add(k, Nat.mul(np, 1n+Nat.add(a, k)))), add_assoc(a, k, Nat.mul(np, 1n+Nat.add(a, k)))) : {False{} == Nat.is_le(1n+_, a) : Bool} internal_div_gt_add(a, Nat.add(k, Nat.mul(np, 1n+Nat.add(a, k))))def internal_div_le_step(+np: Nat, +a: Nat, +bp: Nat, b: Bool, eb: {b == Nat.is_le(1n+bp, a) : Bool}, ih: @x:Nat -> {Nat.is_le(np, Nat.div(x, 1n+bp)) == Nat.is_le(Nat.mul(np, 1n+bp), x) : Bool}) -> {Nat.is_le(1n+np, Nat.div(a, 1n+bp)) == Nat.is_le(Nat.mul(1n+np, 1n+bp), a) : Bool}: match b: case True{}: internal_div_le_big(np, a, bp, internal_div_le_wit(1n+bp, a, Equal.sym(Bool, True{}, Nat.is_le(1n+bp, a), eb)), ih) case False{}: internal_div_le_small(np, a, bp, internal_div_le_wit(a, bp, internal_div_not_succ_le(a, bp, eb)))# A quotient is compared by multiplying back: n <= a / (1 + b) tests as n * (1 + b) <= a.law le_div_iff_mul_le: for n: Nat for +a: Nat for +b: Nat {Nat.is_le(n, Nat.div(a, 1n+b)) == Nat.is_le(Nat.mul(n, 1n+b), a) : Bool}def le_div_iff_mul_le(n, a, b): match n: case 0n: %internal_div_zero_le(Nat.div(a, 1n+b)) : {_ == Nat.is_le(0n, a) : Bool} %internal_div_zero_le(a) : {True{} == _ : Bool} {==} case 1n++np: internal_div_le_step(np, a, b, Nat.is_le(1n+b, a), {==}, x => le_div_iff_mul_le(np, x, b))# Minimum is commutative.law min_comm: for a: Nat for b: Nat {Nat.min(a, b) == Nat.min(b, a) : Nat}def min_comm(a, b): match a b: case 0n 0n: {==} case 0n 1n+q: {==} case 1n+p 0n: {==} case 1n++p 1n++q: %min_comm(p, q) : {1n+Nat.min(p, q) == 1n+_ : Nat} {==}# Maximum is commutative.law max_comm: for a: Nat for b: Nat {Nat.max(a, b) == Nat.max(b, a) : Nat}def max_comm(a, b): match a b: case 0n 0n: {==} case 0n 1n+q: {==} case 1n+p 0n: {==} case 1n++p 1n++q: %max_comm(p, q) : {1n+Nat.max(p, q) == 1n+_ : Nat} {==}# The minimum of a natural and itself is itself.law min_self: for a: Nat {Nat.min(a, a) == a : Nat}def min_self(a): match a: case 0n: {==} case 1n++p: %min_self(p) : {1n+Nat.min(p, p) == 1n+_ : Nat} {==}# The maximum of a natural and itself is itself.law max_self: for a: Nat {Nat.max(a, a) == a : Nat}def max_self(a): match a: case 0n: {==} case 1n++p: %max_self(p) : {1n+Nat.max(p, p) == 1n+_ : Nat} {==}# The minimum with zero is zero: min(a, 0) = 0.law min_zero: for a: Nat {Nat.min(a, 0n) == 0n : Nat}def min_zero(a): match a: case 0n: {==} case 1n+p: {==}# The minimum with zero is zero: min(0, a) = 0.law zero_min: for a: Nat {Nat.min(0n, a) == 0n : Nat}def zero_min(a): match a: case 0n: {==} case 1n+p: {==}# Zero is an identity for maximum: max(a, 0) = a.law max_zero: for a: Nat {Nat.max(a, 0n) == a : Nat}def max_zero(a): match a: case 0n: {==} case 1n+p: {==}# Zero is an identity for maximum: max(0, a) = a.law zero_max: for a: Nat {Nat.max(0n, a) == a : Nat}def zero_max(a): match a: case 0n: {==} case 1n+p: {==}# Minimum is associative.law min_assoc: for a: Nat for b: Nat for c: Nat {Nat.min(Nat.min(a, b), c) == Nat.min(a, Nat.min(b, c)) : Nat}def min_assoc(a, b, c): match a b c: case 0n _ _: {==} case 1n+p 0n _: {==} case 1n+p 1n+q 0n: {==} case 1n++p 1n++q 1n++r: %min_assoc(p, q, r) : {1n+Nat.min(Nat.min(p, q), r) == 1n+_ : Nat} {==}# Maximum is associative.law max_assoc: for a: Nat for b: Nat for c: Nat {Nat.max(Nat.max(a, b), c) == Nat.max(a, Nat.max(b, c)) : Nat}def max_assoc(a, b, c): match a b c: case 0n _ _: {==} case 1n+p 0n _: {==} case 1n+p 1n+q 0n: {==} case 1n++p 1n++q 1n++r: %max_assoc(p, q, r) : {1n+Nat.max(Nat.max(p, q), r) == 1n+_ : Nat} {==}# The minimum plus the maximum is the sum: min(a, b) + max(a, b) = a + b.law min_add_max: for a: Nat for b: Nat {Nat.add(Nat.min(a, b), Nat.max(a, b)) == Nat.add(a, b) : Nat}def min_add_max(a, b): match a b: case 0n 0n: {==} case 0n 1n+q: {==} case 1n++p 0n: %add_zero(p) : {1n+_ == 1n+Nat.add(p, 0n) : Nat} {==} case 1n++p 1n++q: %Equal.sym(Nat, Nat.add(Nat.min(p, q), 1n+Nat.max(p, q)), 1n+Nat.add(Nat.min(p, q), Nat.max(p, q)), add_succ(Nat.min(p, q), Nat.max(p, q))) : {1n+_ == 1n+Nat.add(p, 1n+q) : Nat} %Equal.sym(Nat, Nat.add(p, 1n+q), 1n+Nat.add(p, q), add_succ(p, q)) : {2n+Nat.add(Nat.min(p, q), Nat.max(p, q)) == 1n+_ : Nat} %min_add_max(p, q) : {2n+Nat.add(Nat.min(p, q), Nat.max(p, q)) == 2n+_ : Nat} {==}# The minimum is at most its left argument.law min_le_left: for a: Nat for b: Nat le(Nat.min(a, b), a)def min_le_left(a, b): match a b: case 0n 0n: {==} case 0n 1n+q: {==} case 1n+p 0n: {==} case 1n+p 1n+q: min_le_left(p, q)# The minimum is at most its right argument.law min_le_right: for a: Nat for b: Nat le(Nat.min(a, b), b)def min_le_right(a, b): match a b: case 0n 0n: {==} case 0n 1n+q: {==} case 1n+p 0n: {==} case 1n+p 1n+q: min_le_right(p, q)# The left argument is at most the maximum.law le_max_left: for a: Nat for b: Nat le(a, Nat.max(a, b))def le_max_left(a, b): match a b: case 0n 0n: {==} case 0n 1n+q: {==} case 1n+p 0n: le_refl(1n+p) case 1n+p 1n+q: le_max_left(p, q)# The right argument is at most the maximum.law le_max_right: for a: Nat for b: Nat le(b, Nat.max(a, b))def le_max_right(a, b): match a b: case 0n 0n: {==} case 0n 1n+q: le_refl(1n+q) case 1n+p 0n: {==} case 1n+p 1n+q: le_max_right(p, q)# Any natural to the power zero is one.law pow_zero: for -a: Nat {Nat.pow(a, 0n) == 1n : Nat}def pow_zero(a): {==}# A power with a successor exponent: a^(n+1) = a * a^n.law pow_succ: for -a: Nat for -n: Nat {Nat.pow(a, 1n+n) == Nat.mul(a, Nat.pow(a, n)) : Nat}def pow_succ(a, n): {==}# Any natural to the power one is itself.law pow_one: for a: Nat {Nat.pow(a, 1n) == a : Nat}def pow_one(a): mul_one(a)# One to any power is one.law one_pow: for n: Nat {Nat.pow(1n, n) == 1n : Nat}def one_pow(n): match n: case 0n: {==} case 1n++p: %Equal.sym(Nat, Nat.mul(1n, Nat.pow(1n, p)), Nat.pow(1n, p), one_mul(Nat.pow(1n, p))) : {_ == 1n : Nat} one_pow(p)# Exponents add under multiplication: a^(m+n) = a^m * a^n.law pow_add: for a: Nat for m: Nat for n: Nat {Nat.pow(a, Nat.add(m, n)) == Nat.mul(Nat.pow(a, m), Nat.pow(a, n)) : Nat}def pow_add(a, m, n): match m: case 0n: +a = a +n = n %Equal.sym(Nat, Nat.mul(1n, Nat.pow(a, n)), Nat.pow(a, n), one_mul(Nat.pow(a, n))) : {Nat.pow(a, n) == _ : Nat} {==} case 1n++p: +a = a +n = n %Equal.sym(Nat, Nat.pow(a, Nat.add(p, n)), Nat.mul(Nat.pow(a, p), Nat.pow(a, n)), pow_add(a, p, n)) : {Nat.mul(a, _) == Nat.mul(Nat.mul(a, Nat.pow(a, p)), Nat.pow(a, n)) : Nat} Equal.sym(Nat, Nat.mul(Nat.mul(a, Nat.pow(a, p)), Nat.pow(a, n)), Nat.mul(a, Nat.mul(Nat.pow(a, p), Nat.pow(a, n))), mul_assoc(a, Nat.pow(a, p), Nat.pow(a, n)))# Doubling is adding a natural to itself.law double_eq_add: for n: Nat {Nat.double(n) == Nat.add(n, n) : Nat}def double_eq_add(n): match n: case 0n: {==} case 1n++p: %Equal.sym(Nat, Nat.add(p, 1n+p), 1n+Nat.add(p, p), add_succ(p, p)) : {2n+Nat.double(p) == 1n+_ : Nat} %double_eq_add(p) : {2n+Nat.double(p) == 2n+_ : Nat} {==}# Every natural tests equal to itself.law is_eq_refl: for n: Nat {Nat.is_eq(n, n) == True{} : Bool}def is_eq_refl(n): match n: case 0n: {==} case 1n+p: is_eq_refl(p)# The equality test is symmetric.law is_eq_comm: for a: Nat for b: Nat {Nat.is_eq(a, b) == Nat.is_eq(b, a) : Bool}def is_eq_comm(a, b): match a b: case 0n 0n: {==} case 0n 1n+q: {==} case 1n+p 0n: {==} case 1n++p 1n++q: %is_eq_comm(p, q) : {Nat.is_eq(p, q) == _ : Bool} {==}# If the equality test says true, the naturals are equal.law eq_of_is_eq: for a: Nat for b: Nat for h: {Nat.is_eq(a, b) == True{} : Bool} {a == b : Nat}def eq_of_is_eq(a, b, h): match a b: case 0n 0n: {==} case 0n 1n+q: Empty.absurd({0n == 1n+q : Nat}, internal_false_ne_true(h)) case 1n+p 0n: Empty.absurd({1n+p == 0n : Nat}, internal_false_ne_true(h)) case 1n++p 1n++q: %eq_of_is_eq(p, q, h) : {1n+p == 1n+_ : Nat} {==}# A >= b tests the same as b <= a.law is_ge_eq_is_le: for a: Nat for b: Nat {Nat.is_ge(a, b) == Nat.is_le(b, a) : Bool}def is_ge_eq_is_le(a, b): match a b: case 0n 0n: {==} case 0n 1n+q: {==} case 1n+p 0n: {==} case 1n++p 1n++q: %is_ge_eq_is_le(p, q) : {Nat.is_ge(p, q) == _ : Bool} {==}# A > b tests the same as b < a.law is_gt_eq_is_lt: for a: Nat for b: Nat {Nat.is_gt(a, b) == Nat.is_lt(b, a) : Bool}def is_gt_eq_is_lt(a, b): match a b: case 0n 0n: {==} case 0n 1n+q: {==} case 1n+p 0n: {==} case 1n++p 1n++q: %is_gt_eq_is_lt(p, q) : {Nat.is_gt(p, q) == _ : Bool} {==}# A < b tests the same as a + 1 <= b.law is_lt_eq_succ_le: for a: Nat for b: Nat {Nat.is_lt(a, b) == Nat.is_le(1n+a, b) : Bool}def is_lt_eq_succ_le(a, b): match a b: case 0n 0n: {==} case 0n 1n+q: match q: case 0n: {==} case 1n+r: {==} case 1n+p 0n: {==} case 1n+p 1n+q: is_lt_eq_succ_le(p, q)# Not (a <= b) tests the same as b < a.law not_is_le: for a: Nat for b: Nat {Bool.not(Nat.is_le(a, b)) == Nat.is_lt(b, a) : Bool}def not_is_le(a, b): match a b: case 0n 0n: {==} case 0n 1n+q: {==} case 1n+p 0n: {==} case 1n+p 1n+q: not_is_le(p, q)# Not (a < b) tests the same as b <= a.law not_is_lt: for a: Nat for b: Nat {Bool.not(Nat.is_lt(a, b)) == Nat.is_le(b, a) : Bool}def not_is_lt(a, b): match a b: case 0n 0n: {==} case 0n 1n+q: {==} case 1n+p 0n: {==} case 1n+p 1n+q: not_is_lt(p, q)# Every natural is less than its successor: n < n + 1.law lt_succ_self: for n: Nat lt(n, 1n+n)def lt_succ_self(n): match n: case 0n: {==} case 1n+p: lt_succ_self(p)# The successor preserves the order: a <= b implies a + 1 <= b + 1.law succ_le_succ: for -a: Nat for -b: Nat for h: le(a, b) le(1n+a, 1n+b)def succ_le_succ(a, b, h): h# The order of successors is the order of the naturals: a + 1 <= b + 1 implies a <= b.law le_of_succ_le_succ: for -a: Nat for -b: Nat for h: le(1n+a, 1n+b) le(a, b)def le_of_succ_le_succ(a, b, h): h# A < b and b <= c imply a < c.law lt_of_lt_of_le: for a: Nat for b: Nat for c: Nat for ab: lt(a, b) for bc: le(b, c) lt(a, c)def lt_of_lt_of_le(a, b, c, ab, bc): match a b c: case 0n 0n _: Empty.absurd(lt(0n, c), internal_false_ne_true(ab)) case 0n 1n+q 0n: Empty.absurd(lt(0n, 0n), internal_false_ne_true(bc)) case 0n 1n+q 1n+r: {==} case 1n+p 0n _: Empty.absurd(lt(1n+p, c), internal_false_ne_true(ab)) case 1n+p 1n+q 0n: Empty.absurd(lt(1n+p, 0n), internal_false_ne_true(bc)) case 1n+p 1n+q 1n+r: lt_of_lt_of_le(p, q, r, ab, bc)# A <= b and b < c imply a < c.law lt_of_le_of_lt: for a: Nat for b: Nat for c: Nat for ab: le(a, b) for bc: lt(b, c) lt(a, c)def lt_of_le_of_lt(a, b, c, ab, bc): match a b c: case 0n 0n 0n: Empty.absurd(lt(0n, 0n), internal_false_ne_true(bc)) case 0n 1n+q 0n: Empty.absurd(lt(0n, 0n), internal_false_ne_true(bc)) case 0n _ 1n+r: {==} case 1n+p 0n _: Empty.absurd(lt(1n+p, c), internal_false_ne_true(ab)) case 1n+p 1n+q 0n: Empty.absurd(lt(1n+p, 0n), internal_false_ne_true(bc)) case 1n+p 1n+q 1n+r: lt_of_le_of_lt(p, q, r, ab, bc)# Adding on the left preserves the order: a <= b implies k + a <= k + b.law add_le_add_left: for -a: Nat for -b: Nat for k: Nat for h: le(a, b) le(Nat.add(k, a), Nat.add(k, b))def add_le_add_left(a, b, k, h): match k: case 0n: h case 1n+p: add_le_add_left(a, b, p, h)# Adding on the right preserves the order: a <= b implies a + k <= b + k.law add_le_add_right: for +a: Nat for +b: Nat for +k: Nat for h: le(a, b) le(Nat.add(a, k), Nat.add(b, k))def add_le_add_right(a, b, k, h): %add_comm(k, a) : le(_, Nat.add(b, k)) %add_comm(k, b) : le(Nat.add(k, a), _) add_le_add_left(a, b, k, h)# Adding the same amount on the left does not change the order test: a <= b tests as k + a <= k + b.law add_le_add_iff_left: for k: Nat for -a: Nat for -b: Nat {Nat.is_le(a, b) == Nat.is_le(Nat.add(k, a), Nat.add(k, b)) : Bool}def add_le_add_iff_left(k, a, b): match k: case 0n: {==} case 1n+p: add_le_add_iff_left(p, a, b)# Adding two bounded summands stays bounded: a <= c and b <= d imply a + b <= c + d.law add_le_add: for +a: Nat for +b: Nat for +c: Nat for +d: Nat for h1: le(a, c) for h2: le(b, d) le(Nat.add(a, b), Nat.add(c, d))def add_le_add(a, b, c, d, h1, h2): le_trans(Nat.add(a, b), Nat.add(c, b), Nat.add(c, d), add_le_add_right(a, c, b, h1), add_le_add_left(b, d, c, h2))# A sum is at most c exactly when a <= c and b <= c - a: a + b <= c tests as both.law le_and_le_sub_iff_add_le: for a: Nat for -b: Nat for c: Nat {Bool.and(Nat.is_le(a, c), Nat.is_le(b, Nat.sub(c, a))) == Nat.is_le(Nat.add(a, b), c) : Bool}def le_and_le_sub_iff_add_le(a, b, c): match a c: case 0n 0n: {==} case 0n 1n+j: {==} case 1n+i 0n: {==} case 1n+i 1n+j: le_and_le_sub_iff_add_le(i, b, j)# The only natural at most zero is zero.law le_zero_eq: for n: Nat for h: le(n, 0n) {n == 0n : Nat}def le_zero_eq(n, h): match n: case 0n: {==} case 1n+p: Empty.absurd({1n+p == 0n : Nat}, internal_false_ne_true(h))# No natural is less than zero.law lt_zero: for n: Nat lt(n, 0n) -> Emptydef lt_zero(n): match n: case 0n: h => internal_false_ne_true(h) case 1n+p: h => internal_false_ne_true(h)# A strict inequality rules out the reverse weak one: a < b implies b <= a is false.law not_le_of_lt: for a: Nat for b: Nat for h: lt(a, b) {Nat.is_le(b, a) == False{} : Bool}def not_le_of_lt(a, b, h): match a b: case 0n 0n: Empty.absurd({Nat.is_le(0n, 0n) == False{} : Bool}, internal_false_ne_true(h)) case 0n 1n+q: {==} case 1n+p 0n: Empty.absurd({Nat.is_le(0n, 1n+p) == False{} : Bool}, internal_false_ne_true(h)) case 1n+p 1n+q: not_le_of_lt(p, q, h)# A weak inequality rules out the reverse strict one: a <= b implies b < a is false.law not_lt_of_le: for a: Nat for b: Nat for h: le(a, b) {Nat.is_lt(b, a) == False{} : Bool}def not_lt_of_le(a, b, h): match a b: case 0n 0n: {==} case 0n 1n+q: {==} case 1n+p 0n: Empty.absurd({Nat.is_lt(0n, 1n+p) == False{} : Bool}, internal_false_ne_true(h)) case 1n+p 1n+q: not_lt_of_le(p, q, h)# A failed weak test gives the reverse strict order: a <= b false implies b < a.law lt_of_not_le: for a: Nat for b: Nat for h: {Nat.is_le(a, b) == False{} : Bool} lt(b, a)def lt_of_not_le(a, b, h): match a b: case 0n 0n: Empty.absurd(lt(0n, 0n), internal_false_ne_true(Equal.sym(Bool, True{}, False{}, h))) case 0n 1n+q: Empty.absurd(lt(1n+q, 0n), internal_false_ne_true(Equal.sym(Bool, True{}, False{}, h))) case 1n+p 0n: {==} case 1n+p 1n+q: lt_of_not_le(p, q, h)# A failed strict test gives the reverse weak order: a < b false implies b <= a.law le_of_not_lt: for a: Nat for b: Nat for h: {Nat.is_lt(a, b) == False{} : Bool} le(b, a)def le_of_not_lt(a, b, h): match a b: case 0n 0n: {==} case 0n 1n+q: Empty.absurd(le(1n+q, 0n), internal_false_ne_true(Equal.sym(Bool, True{}, False{}, h))) case 1n+p 0n: {==} case 1n+p 1n+q: le_of_not_lt(p, q, h)# A number below both bounds is below their minimum: a < b and a < c imply a < min b c.law lt_min: for a: Nat for b: Nat for c: Nat for hb: lt(a, b) for hc: lt(a, c) lt(a, Nat.min(b, c))def lt_min(a, b, c, hb, hc): match a b c: case _ 0n _: Empty.absurd(lt(a, Nat.min(0n, c)), lt_zero(a)(hb)) case _ 1n+q 0n: Empty.absurd(lt(a, Nat.min(1n+q, 0n)), lt_zero(a)(hc)) case 0n 1n+q 1n+r: {==} case 1n+p 1n+q 1n+r: lt_min(p, q, r, hb, hc)def internal_or_true_sym(x: Bool) -> {True{} == Bool.or(x, True{}) : Bool}: match x: case False{}: {==} case True{}: {==}def internal_or_false_sym(x: Bool) -> {x == Bool.or(x, False{}) : Bool}: match x: case False{}: {==} case True{}: {==}# The minimum is at most c exactly when one argument is: min a b <= c tests as a <= c or b <= c.law min_le_iff: for a: Nat for b: Nat for c: Nat {Nat.is_le(Nat.min(a, b), c) == Bool.or(Nat.is_le(a, c), Nat.is_le(b, c)) : Bool}def min_le_iff(a, b, c): match a b c: case 0n 0n 0n: {==} case 0n 0n 1n+r: {==} case 0n 1n+q 0n: {==} case 0n 1n+q 1n+r: {==} case 1n+p 0n 0n: {==} case 1n+p 0n 1n+r: internal_or_true_sym(Nat.is_le(p, r)) case 1n+p 1n+q 0n: {==} case 1n+p 1n+q 1n+r: min_le_iff(p, q, r)# The maximum is above a exactly when one argument is: a < max b c tests as a < b or a < c.law lt_max_iff: for a: Nat for b: Nat for c: Nat {Nat.is_lt(a, Nat.max(b, c)) == Bool.or(Nat.is_lt(a, b), Nat.is_lt(a, c)) : Bool}def lt_max_iff(a, b, c): match a b c: case 0n 0n 0n: {==} case 0n 0n 1n+r: {==} case 0n 1n+q 0n: {==} case 0n 1n+q 1n+r: {==} case 1n+p 0n 0n: {==} case 1n+p 0n 1n+r: {==} case 1n+p 1n+q 0n: internal_or_false_sym(Nat.is_lt(p, q)) case 1n+p 1n+q 1n+r: lt_max_iff(p, q, r)# Subtracting a larger number gives zero: a <= b implies a - b = 0.law sub_eq_zero_of_le: for a: Nat for b: Nat for h: le(a, b) {Nat.sub(a, b) == 0n : Nat}def sub_eq_zero_of_le(a, b, h): match a b: case 0n 0n: {==} case 0n 1n+q: {==} case 1n+p 0n: Empty.absurd({Nat.sub(1n+p, 0n) == 0n : Nat}, internal_false_ne_true(h)) case 1n+p 1n+q: sub_eq_zero_of_le(p, q, h)# Above the subtrahend, a successor subtracts to a successor: b <= a implies (a + 1) - b = (a - b) + 1.law succ_sub: for a: Nat for b: Nat for h: le(b, a) {Nat.sub(1n+a, b) == 1n+Nat.sub(a, b) : Nat}def succ_sub(a, b, h): match a b: case 0n 0n: {==} case 1n+p 0n: {==} case 0n 1n+q: Empty.absurd({Nat.sub(1n, 1n+q) == 1n+Nat.sub(0n, 1n+q) : Nat}, internal_false_ne_true(h)) case 1n+p 1n+q: succ_sub(p, q, h)# Adding back what was subtracted restores the number: a <= b implies a + (b - a) = b.law add_sub_of_le: for a: Nat for b: Nat for h: le(a, b) {Nat.add(a, Nat.sub(b, a)) == b : Nat}def add_sub_of_le(a, b, h): match a b: case 0n 0n: {==} case 0n 1n+q: {==} case 1n+p 0n: Empty.absurd({Nat.add(1n+p, Nat.sub(0n, 1n+p)) == 0n : Nat}, internal_false_ne_true(h)) case 1n++p 1n++q: %Equal.sym(Nat, Nat.add(p, Nat.sub(q, p)), q, add_sub_of_le(p, q, h)) : {1n+_ == 1n+q : Nat} {==}def internal_lt_sub_zero(a: Nat, p: Nat) -> {Nat.is_lt(a, Nat.sub(0n, 1n+p)) == Nat.is_lt(Nat.add(1n+p, a), 0n) : Bool}: match a: case 0n: {==} case 1n+x: {==}# Comparing against a difference is comparing the sum: a < c - b tests as b + a < c.law lt_sub_iff_add_lt: for a: Nat for b: Nat for c: Nat {Nat.is_lt(a, Nat.sub(c, b)) == Nat.is_lt(Nat.add(b, a), c) : Bool}def lt_sub_iff_add_lt(a, b, c): match b c: case 0n 0n: {==} case 0n 1n+r: {==} case 1n+p 0n: internal_lt_sub_zero(a, p) case 1n+p 1n+r: lt_sub_iff_add_lt(a, p, r)# A witnessed difference gives the order: a + k = b implies a <= b.law le_of_add_eq: for a: Nat for k: Nat for -b: Nat for e: {Nat.add(a, k) == b : Nat} le(a, b)def le_of_add_eq(a, k, b, e): %e : {Nat.is_le(a, _) == True{} : Bool} le_add_right(a, k)# Multiplying on the right preserves the order: a <= b implies a * k <= b * k.law mul_le_mul_right: for +a: Nat for +b: Nat for +k: Nat for h: le(a, b) le(Nat.mul(a, k), Nat.mul(b, k))def mul_le_mul_right(a, b, k, h): %sub_add_cancel(b, a, h) : {Nat.is_le(Nat.mul(a, k), Nat.mul(_, k)) == True{} : Bool} %Equal.sym(Nat, Nat.mul(Nat.add(Nat.sub(b, a), a), k), Nat.add(Nat.mul(Nat.sub(b, a), k), Nat.mul(a, k)), add_mul(Nat.sub(b, a), a, k)) : {Nat.is_le(Nat.mul(a, k), _) == True{} : Bool} %add_comm(Nat.mul(a, k), Nat.mul(Nat.sub(b, a), k)) : {Nat.is_le(Nat.mul(a, k), _) == True{} : Bool} le_add_right(Nat.mul(a, k), Nat.mul(Nat.sub(b, a), k))# Zero divided by anything is zero: 0 / b = 0.law zero_div: for b: Nat {Nat.div(0n, b) == 0n : Nat}def zero_div(b): match b: case 0n: {==} case 1n+p: {==}# Zero modulo anything is zero: 0 % b = 0.law zero_mod: for b: Nat {Nat.mod(0n, b) == 0n : Nat}def zero_mod(b): match b: case 0n: {==} case 1n+p: {==}def internal_div_go_one(a: Nat, +d: Nat) -> {(Nat.add(a, d), 0n) == Nat.divmod.go(a, 0n, d, 0n) : Nat & Nat}: match a: case 0n: {==} case 1n++p: %add_succ(p, d) : {(_, 0n) == Nat.divmod.go(p, 0n, 1n+d, 0n) : Nat & Nat} internal_div_go_one(p, 1n+d)# Dividing by one changes nothing: a / 1 = a.law div_one: for a: Nat {Nat.div(a, 1n) == a : Nat}def div_one(a): +a = a %internal_div_go_one(a, 0n) : {Pair.fst(Nat, Nat, _) == a : Nat} add_zero(a)# Any natural modulo one is zero: a % 1 = 0.law mod_one: for a: Nat {Nat.mod(a, 1n) == 0n : Nat}def mod_one(a): +a = a %internal_div_go_one(a, 0n) : {Pair.snd(Nat, Nat, _) == 0n : Nat} {==}def internal_div_go_self(m: Nat, -d: Nat, -r: Nat) -> {(1n+d, 0n) == Nat.divmod.go(1n+m, m, d, r) : Nat & Nat}: match m: case 0n: {==} case 1n+p: internal_div_go_self(p, d, 1n+r)# A natural modulo itself is zero: n % n = 0.law mod_self: for n: Nat {Nat.mod(n, n) == 0n : Nat}def mod_self(n): match n: case 0n: {==} case 1n++p: %internal_div_go_self(p, 0n, 0n) : {Pair.snd(Nat, Nat, _) == 0n : Nat} {==}# A positive natural divided by itself is one: (1 + b) / (1 + b) = 1.law div_self: for b: Nat {Nat.div(1n+b, 1n+b) == 1n : Nat}def div_self(b): +b = b %internal_div_go_self(b, 0n, 0n) : {Pair.fst(Nat, Nat, _) == 1n : Nat} {==}def internal_mod_go_le(n: Nat, m: Nat, -d: Nat, +r: Nat) -> le(Pair.snd(Nat, Nat, Nat.divmod.go(n, m, d, r)), Nat.add(r, m)): match n m: case 0n k: le_add_right(r, k) case 1n++np 0n: %internal_div_add_zero_r(r) : le(Pair.snd(Nat, Nat, Nat.divmod.go(np, r, 1n+d, 0n)), _) internal_mod_go_le(np, r, 1n+d, 0n) case 1n++np 1n++mp: %Equal.sym(Nat, Nat.add(r, 1n+mp), 1n+Nat.add(r, mp), add_succ(r, mp)) : le(Pair.snd(Nat, Nat, Nat.divmod.go(np, mp, d, 1n+r)), _) internal_mod_go_le(np, mp, d, 1n+r)# A remainder is below its positive divisor: a % (1 + b) < 1 + b.law mod_lt: for a: Nat for b: Nat lt(Nat.mod(a, 1n+b), 1n+b)def mod_lt(a, b): +a = a +b = b %Equal.sym(Bool, Nat.is_lt(Nat.mod(a, 1n+b), 1n+b), Nat.is_le(1n+Nat.mod(a, 1n+b), 1n+b), is_lt_eq_succ_le(Nat.mod(a, 1n+b), 1n+b)) : {_ == True{} : Bool} internal_mod_go_le(a, b, 0n, 0n)# A remainder never exceeds the dividend: a % b <= a.law mod_le: for a: Nat for b: Nat le(Nat.mod(a, b), a)def mod_le(a, b): match b: case 0n: le_refl(a) case 1n++bp: +a = a +bp = bp le_of_add_eq(Nat.mod(a, 1n+bp), Nat.mul(Nat.div(a, 1n+bp), 1n+bp), a, Equal.trans(Nat, Nat.add(Nat.mod(a, 1n+bp), Nat.mul(Nat.div(a, 1n+bp), 1n+bp)), Nat.add(Nat.mul(Nat.div(a, 1n+bp), 1n+bp), Nat.mod(a, 1n+bp)), a, add_comm(Nat.mod(a, 1n+bp), Nat.mul(Nat.div(a, 1n+bp), 1n+bp)), div_mod_eq(a, bp)))def internal_div_le_self_eq(+q: Nat, +bp: Nat, +m: Nat, -a: Nat, e: {Nat.add(Nat.mul(q, 1n+bp), m) == a : Nat}) -> {Nat.add(q, Nat.add(Nat.mul(bp, q), m)) == a : Nat}: %e : {Nat.add(q, Nat.add(Nat.mul(bp, q), m)) == _ : Nat} %mul_comm(1n+bp, q) : {Nat.add(q, Nat.add(Nat.mul(bp, q), m)) == Nat.add(_, m) : Nat} Equal.sym(Nat, Nat.add(Nat.add(q, Nat.mul(bp, q)), m), Nat.add(q, Nat.add(Nat.mul(bp, q), m)), add_assoc(q, Nat.mul(bp, q), m))# A quotient never exceeds the dividend: a / b <= a.law div_le_self: for a: Nat for b: Nat le(Nat.div(a, b), a)def div_le_self(a, b): match b: case 0n: zero_le(a) case 1n++bp: +a = a +bp = bp le_of_add_eq(Nat.div(a, 1n+bp), Nat.add(Nat.mul(bp, Nat.div(a, 1n+bp)), Nat.mod(a, 1n+bp)), a, internal_div_le_self_eq(Nat.div(a, 1n+bp), bp, Nat.mod(a, 1n+bp), a, div_mod_eq(a, bp)))def internal_mod_go_small(n: Nat, -k: Nat, -d: Nat, +r: Nat) -> {Pair.snd(Nat, Nat, Nat.divmod.go(n, Nat.add(n, k), d, r)) == Nat.add(n, r) : Nat}: match n: case 0n: {==} case 1n++np: %add_succ(np, r) : {Pair.snd(Nat, Nat, Nat.divmod.go(np, Nat.add(np, k), d, 1n+r)) == _ : Nat} internal_mod_go_small(np, k, d, 1n+r)def internal_mod_eq_of_wit(+a: Nat, +b: Nat, w: &k:Nat -> {Nat.add(a, k) == b : Nat}) -> {Nat.mod(a, 1n+b) == a : Nat}: match w: case (+k, e): %e : {Nat.mod(a, 1n+_) == a : Nat} Equal.trans(Nat, Nat.mod(a, 1n+Nat.add(a, k)), Nat.add(a, 0n), a, internal_mod_go_small(a, k, 0n, 0n), add_zero(a))# A number below the divisor is its own remainder: a < b implies a % b = a.law mod_eq_of_lt: for a: Nat for b: Nat for h: lt(a, b) {Nat.mod(a, b) == a : Nat}def mod_eq_of_lt(a, b, h): match b: case 0n: +a = a Empty.absurd({Nat.mod(a, 0n) == a : Nat}, lt_zero(a)(h)) case 1n++bp: +a = a +bp = bp internal_mod_eq_of_wit(a, bp, internal_div_le_wit(a, bp, Equal.trans(Bool, Nat.is_le(1n+a, 1n+bp), Nat.is_lt(a, 1n+bp), True{}, Equal.sym(Bool, Nat.is_lt(a, 1n+bp), Nat.is_le(1n+a, 1n+bp), is_lt_eq_succ_le(a, 1n+bp)), h)))# Taking a remainder twice is taking it once: (a % n) % n = a % n.law mod_mod: for a: Nat for n: Nat {Nat.mod(Nat.mod(a, n), n) == Nat.mod(a, n) : Nat}def mod_mod(a, n): match n: case 0n: {==} case 1n++b: +a = a +b = b mod_eq_of_lt(Nat.mod(a, 1n+b), 1n+b, mod_lt(a, b))def internal_mod_go_d(n: Nat, m: Nat, -d: Nat, -e: Nat, r: Nat) -> {Pair.snd(Nat, Nat, Nat.divmod.go(n, m, d, r)) == Pair.snd(Nat, Nat, Nat.divmod.go(n, m, e, r)) : Nat}: match n m: case 0n _: {==} case 1n+np 0n: internal_mod_go_d(np, r, 1n+d, 1n+e, 0n) case 1n+np 1n+mp: internal_mod_go_d(np, mp, d, e, 1n+r)# Adding the divisor does not change the remainder: (b + a) % b = a % b.law add_mod_left: for a: Nat for b: Nat {Nat.mod(Nat.add(b, a), b) == Nat.mod(a, b) : Nat}def add_mod_left(a, b): match b: case 0n: {==} case 1n++bp: +a = a %internal_div_go_run(bp, a, 0n, 0n) : {Pair.snd(Nat, Nat, _) == Nat.mod(a, 1n+bp) : Nat} %internal_div_add_zero_r(bp) : {Pair.snd(Nat, Nat, Nat.divmod.go(a, _, 1n, 0n)) == Nat.mod(a, 1n+bp) : Nat} internal_mod_go_d(a, bp, 1n, 0n, 0n)# Multiplying by a positive divisor then dividing by it cancels: (a * (1 + b)) / (1 + b) = a.law mul_div_cancel: for a: Nat for b: Nat {Nat.div(Nat.mul(a, 1n+b), 1n+b) == a : Nat}def mul_div_cancel(a, b): match a: case 0n: {==} case 1n++p: +b = b %Equal.sym(Nat, Nat.div(Nat.add(1n+b, Nat.mul(p, 1n+b)), 1n+b), 1n+Nat.div(Nat.mul(p, 1n+b), 1n+b), add_div_left(Nat.mul(p, 1n+b), b)) : {_ == 1n+p : Nat} %mul_div_cancel(p, b) : {1n+Nat.div(Nat.mul(p, 1n+b), 1n+b) == 1n+_ : Nat} {==}# Multiplying on the left by a positive divisor then dividing by it cancels: ((1 + b) * a) / (1 + b) = a.law mul_div_cancel_left: for a: Nat for b: Nat {Nat.div(Nat.mul(1n+b, a), 1n+b) == a : Nat}def mul_div_cancel_left(a, b): +a = a +b = b %mul_comm(a, 1n+b) : {Nat.div(_, 1n+b) == a : Nat} mul_div_cancel(a, b)# A multiple of b leaves no remainder modulo b: (a * b) % b = 0.law mul_mod_left: for a: Nat for b: Nat {Nat.mod(Nat.mul(a, b), b) == 0n : Nat}def mul_mod_left(a, b): match a b: case _ 0n: mul_zero(a) case 0n 1n+bp: {==} case 1n++p 1n++bp: Equal.trans(Nat, Nat.mod(Nat.add(1n+bp, Nat.mul(p, 1n+bp)), 1n+bp), Nat.mod(Nat.mul(p, 1n+bp), 1n+bp), 0n, add_mod_left(Nat.mul(p, 1n+bp), 1n+bp), mul_mod_left(p, 1n+bp))# A multiple of a leaves no remainder modulo a: (a * b) % a = 0.law mul_mod_right: for a: Nat for b: Nat {Nat.mod(Nat.mul(a, b), a) == 0n : Nat}def mul_mod_right(a, b): +a = a +b = b %mul_comm(b, a) : {Nat.mod(_, a) == 0n : Nat} mul_mod_left(b, a)# The remainder plus the divisor times the quotient is the dividend: a % b + b * (a / b) = a.law mod_add_div: for a: Nat for b: Nat {Nat.add(Nat.mod(a, b), Nat.mul(b, Nat.div(a, b))) == a : Nat}def mod_add_div(a, b): match b: case 0n: add_zero(a) case 1n++bp: +a = a %add_comm(Nat.mul(1n+bp, Nat.div(a, 1n+bp)), Nat.mod(a, 1n+bp)) : {_ == a : Nat} %mul_comm(Nat.div(a, 1n+bp), 1n+bp) : {Nat.add(_, Nat.mod(a, 1n+bp)) == a : Nat} div_mod_eq(a, bp)# The quotient times the divisor never exceeds the dividend: (a / b) * b <= a.law div_mul_le_self: for a: Nat for b: Nat le(Nat.mul(Nat.div(a, b), b), a)def div_mul_le_self(a, b): match b: case 0n: zero_le(a) case 1n++bp: +a = a le_of_add_eq(Nat.mul(Nat.div(a, 1n+bp), 1n+bp), Nat.mod(a, 1n+bp), a, div_mod_eq(a, bp))# Left commutativity of multiplication: a * (b * c) = b * (a * c).law mul_left_comm: for a: Nat for b: Nat for c: Nat {Nat.mul(a, Nat.mul(b, c)) == Nat.mul(b, Nat.mul(a, c)) : Nat}def mul_left_comm(a, b, c): +a = a +b = b +c = c %mul_assoc(a, b, c) : {_ == Nat.mul(b, Nat.mul(a, c)) : Nat} %mul_comm(b, a) : {Nat.mul(_, c) == Nat.mul(b, Nat.mul(a, c)) : Nat} mul_assoc(b, a, c)# Right commutativity of multiplication: (a * b) * c = (a * c) * b.law mul_right_comm: for a: Nat for b: Nat for c: Nat {Nat.mul(Nat.mul(a, b), c) == Nat.mul(Nat.mul(a, c), b) : Nat}def mul_right_comm(a, b, c): +a = a +b = b +c = c %Equal.sym(Nat, Nat.mul(Nat.mul(a, c), b), Nat.mul(a, Nat.mul(c, b)), mul_assoc(a, c, b)) : {Nat.mul(Nat.mul(a, b), c) == _ : Nat} %mul_comm(b, c) : {Nat.mul(Nat.mul(a, b), c) == Nat.mul(a, _) : Nat} mul_assoc(a, b, c)# Four-way regrouping of a product: (a * b) * (c * d) = (a * c) * (b * d).law mul_mul_mul_comm: for a: Nat for b: Nat for c: Nat for d: Nat {Nat.mul(Nat.mul(a, b), Nat.mul(c, d)) == Nat.mul(Nat.mul(a, c), Nat.mul(b, d)) : Nat}def mul_mul_mul_comm(a, b, c, d): +a = a +b = b +c = c +d = d %Equal.sym(Nat, Nat.mul(Nat.mul(a, c), Nat.mul(b, d)), Nat.mul(a, Nat.mul(c, Nat.mul(b, d))), mul_assoc(a, c, Nat.mul(b, d))) : {Nat.mul(Nat.mul(a, b), Nat.mul(c, d)) == _ : Nat} %mul_left_comm(b, c, d) : {Nat.mul(Nat.mul(a, b), Nat.mul(c, d)) == Nat.mul(a, _) : Nat} mul_assoc(a, b, Nat.mul(c, d))# Zero to a positive power is zero: 0^(n+1) = 0.law zero_pow: for -n: Nat {Nat.pow(0n, 1n+n) == 0n : Nat}def zero_pow(n): {==}# A power of a product is the product of the powers: (a * b)^n = a^n * b^n.law mul_pow: for +a: Nat for +b: Nat for n: Nat {Nat.pow(Nat.mul(a, b), n) == Nat.mul(Nat.pow(a, n), Nat.pow(b, n)) : Nat}def mul_pow(a, b, n): match n: case 0n: {==} case 1n++p: %Equal.sym(Nat, Nat.pow(Nat.mul(a, b), p), Nat.mul(Nat.pow(a, p), Nat.pow(b, p)), mul_pow(a, b, p)) : {Nat.mul(Nat.mul(a, b), _) == Nat.mul(Nat.mul(a, Nat.pow(a, p)), Nat.mul(b, Nat.pow(b, p))) : Nat} mul_mul_mul_comm(a, b, Nat.pow(a, p), Nat.pow(b, p))# Exponents multiply under repeated powers: a^(m * n) = (a^m)^n.law pow_mul: for a: Nat for m: Nat for n: Nat {Nat.pow(a, Nat.mul(m, n)) == Nat.pow(Nat.pow(a, m), n) : Nat}def pow_mul(a, m, n): match m: case 0n: Equal.sym(Nat, Nat.pow(1n, n), 1n, one_pow(n)) case 1n++p: +a = a +n = n %Equal.sym(Nat, Nat.pow(Nat.mul(a, Nat.pow(a, p)), n), Nat.mul(Nat.pow(a, n), Nat.pow(Nat.pow(a, p), n)), mul_pow(a, Nat.pow(a, p), n)) : {Nat.pow(a, Nat.add(n, Nat.mul(p, n))) == _ : Nat} %pow_mul(a, p, n) : {Nat.pow(a, Nat.add(n, Nat.mul(p, n))) == Nat.mul(Nat.pow(a, n), _) : Nat} pow_add(a, n, Nat.mul(p, n))# A power of a positive base is at least one: 1 <= (1 + a)^n.law one_le_pow: for n: Nat for a: Nat le(1n, Nat.pow(1n+a, n))def one_le_pow(n, a): match n: case 0n: {==} case 1n++p: +a = a le_trans(1n, Nat.pow(1n+a, p), Nat.add(Nat.pow(1n+a, p), Nat.mul(a, Nat.pow(1n+a, p))), one_le_pow(p, a), le_add_right(Nat.pow(1n+a, p), Nat.mul(a, Nat.pow(1n+a, p))))# A power of a positive base is positive: 0 < (1 + a)^n.law pow_pos: for n: Nat for a: Nat lt(0n, Nat.pow(1n+a, n))def pow_pos(n, a): +n = n +a = a %Equal.sym(Bool, Nat.is_lt(0n, Nat.pow(1n+a, n)), Nat.is_le(1n, Nat.pow(1n+a, n)), is_lt_eq_succ_le(0n, Nat.pow(1n+a, n))) : {_ == True{} : Bool} one_le_pow(n, a)# Adding on the left never decreases a natural: n <= m + n.law le_add_left: for n: Nat for m: Nat le(n, Nat.add(m, n))def le_add_left(n, m): match m: case 0n: le_refl(n) case 1n++p: +n = n le_trans(n, Nat.add(p, n), 1n+Nat.add(p, n), le_add_left(n, p), le_succ(Nat.add(p, n)))# A common left summand cancels in a difference: (k + n) - (k + m) = n - m.law add_sub_add_left: for k: Nat for -n: Nat for -m: Nat {Nat.sub(Nat.add(k, n), Nat.add(k, m)) == Nat.sub(n, m) : Nat}def add_sub_add_left(k, n, m): match k: case 0n: {==} case 1n+p: add_sub_add_left(p, n, m)# A common right summand cancels in a difference: (n + k) - (m + k) = n - m.law add_sub_add_right: for n: Nat for k: Nat for m: Nat {Nat.sub(Nat.add(n, k), Nat.add(m, k)) == Nat.sub(n, m) : Nat}def add_sub_add_right(n, k, m): +n = n +k = k +m = m %add_comm(k, n) : {Nat.sub(_, Nat.add(m, k)) == Nat.sub(n, m) : Nat} %add_comm(k, m) : {Nat.sub(Nat.add(k, n), _) == Nat.sub(n, m) : Nat} add_sub_add_left(k, n, m)# Subtracting a part of the right summand: k <= m implies (n + m) - k = n + (m - k).law add_sub_assoc: for k: Nat for m: Nat for h: le(k, m) for n: Nat {Nat.sub(Nat.add(n, m), k) == Nat.add(n, Nat.sub(m, k)) : Nat}def add_sub_assoc(k, m, h, n): +k = k +m = m +n = n %sub_add_cancel(m, k, h) : {Nat.sub(Nat.add(n, _), k) == Nat.add(n, Nat.sub(m, k)) : Nat} %add_assoc(n, Nat.sub(m, k), k) : {Nat.sub(_, k) == Nat.add(n, Nat.sub(m, k)) : Nat} add_sub_cancel(Nat.add(n, Nat.sub(m, k)), k)# Subtracting a difference from its minuend: m <= n implies n - (n - m) = m.law sub_sub_self: for n: Nat for m: Nat for h: le(m, n) {Nat.sub(n, Nat.sub(n, m)) == m : Nat}def sub_sub_self(n, m, h): match n m: case 0n 0n: {==} case 0n 1n+q: Empty.absurd({Nat.sub(0n, Nat.sub(0n, 1n+q)) == 1n+q : Nat}, internal_false_ne_true(h)) case 1n+p 0n: sub_self(p) case 1n++p 1n++q: Equal.trans(Nat, Nat.sub(1n+p, Nat.sub(p, q)), 1n+Nat.sub(p, Nat.sub(p, q)), 1n+q, succ_sub(p, Nat.sub(p, q), sub_le(p, q)), Equal.cong(Nat, Nat, x => 1n+x, Nat.sub(p, Nat.sub(p, q)), q, sub_sub_self(p, q, h)))# Multiplication distributes over subtraction on the right: (n - m) * k = n * k - m * k.law sub_mul: for n: Nat for m: Nat for k: Nat {Nat.mul(Nat.sub(n, m), k) == Nat.sub(Nat.mul(n, k), Nat.mul(m, k)) : Nat}def sub_mul(n, m, k): match n m: case 0n _: {==} case 1n++p 0n: +k = k Equal.sym(Nat, Nat.sub(Nat.mul(1n+p, k), 0n), Nat.mul(1n+p, k), sub_zero(Nat.mul(1n+p, k))) case 1n++p 1n++q: +k = k %Equal.sym(Nat, Nat.sub(Nat.add(k, Nat.mul(p, k)), Nat.add(k, Nat.mul(q, k))), Nat.sub(Nat.mul(p, k), Nat.mul(q, k)), add_sub_add_left(k, Nat.mul(p, k), Nat.mul(q, k))) : {Nat.mul(Nat.sub(p, q), k) == _ : Nat} sub_mul(p, q, k)# Multiplication distributes over subtraction on the left: n * (m - k) = n * m - n * k.law mul_sub: for n: Nat for m: Nat for k: Nat {Nat.mul(n, Nat.sub(m, k)) == Nat.sub(Nat.mul(n, m), Nat.mul(n, k)) : Nat}def mul_sub(n, m, k): +n = n +m = m +k = k %mul_comm(Nat.sub(m, k), n) : {_ == Nat.sub(Nat.mul(n, m), Nat.mul(n, k)) : Nat} %mul_comm(m, n) : {Nat.mul(Nat.sub(m, k), n) == Nat.sub(_, Nat.mul(n, k)) : Nat} %mul_comm(k, n) : {Nat.mul(Nat.sub(m, k), n) == Nat.sub(Nat.mul(m, n), _) : Nat} sub_mul(m, k, n)# The minimum is the smaller argument on the left: a <= b implies min a b = a.law min_eq_left: for a: Nat for b: Nat for h: le(a, b) {Nat.min(a, b) == a : Nat}def min_eq_left(a, b, h): match a b: case 0n _: {==} case 1n+p 0n: Empty.absurd({Nat.min(1n+p, 0n) == 1n+p : Nat}, internal_false_ne_true(h)) case 1n++p 1n+q: %min_eq_left(p, q, h) : {1n+Nat.min(p, q) == 1n+_ : Nat} {==}# The minimum is the smaller argument on the right: b <= a implies min a b = b.law min_eq_right: for a: Nat for b: Nat for h: le(b, a) {Nat.min(a, b) == b : Nat}def min_eq_right(a, b, h): match a b: case 0n 0n: {==} case 0n 1n+q: Empty.absurd({Nat.min(0n, 1n+q) == 1n+q : Nat}, internal_false_ne_true(h)) case 1n+p 0n: {==} case 1n+p 1n++q: %min_eq_right(p, q, h) : {1n+Nat.min(p, q) == 1n+_ : Nat} {==}# The maximum is the larger argument on the left: b <= a implies max a b = a.law max_eq_left: for a: Nat for b: Nat for h: le(b, a) {Nat.max(a, b) == a : Nat}def max_eq_left(a, b, h): match a b: case 0n 0n: {==} case 0n 1n+q: Empty.absurd({Nat.max(0n, 1n+q) == 0n : Nat}, internal_false_ne_true(h)) case 1n+p 0n: {==} case 1n++p 1n+q: %max_eq_left(p, q, h) : {1n+Nat.max(p, q) == 1n+_ : Nat} {==}# The maximum is the larger argument on the right: a <= b implies max a b = b.law max_eq_right: for a: Nat for b: Nat for h: le(a, b) {Nat.max(a, b) == b : Nat}def max_eq_right(a, b, h): match a b: case 0n _: {==} case 1n+p 0n: Empty.absurd({Nat.max(1n+p, 0n) == 0n : Nat}, internal_false_ne_true(h)) case 1n+p 1n++q: %max_eq_right(p, q, h) : {1n+Nat.max(p, q) == 1n+_ : Nat} {==}def internal_and_false_sym(x: Bool) -> {False{} == Bool.and(x, False{}) : Bool}: match x: case False{}: {==} case True{}: {==}def internal_and_true_sym(x: Bool) -> {x == Bool.and(x, True{}) : Bool}: match x: case False{}: {==} case True{}: {==}# A number is at most the minimum exactly when it is at most both: a <= min b c tests as a <= b and a <= c.law le_min: for a: Nat for b: Nat for c: Nat {Nat.is_le(a, Nat.min(b, c)) == Bool.and(Nat.is_le(a, b), Nat.is_le(a, c)) : Bool}def le_min(a, b, c): match a b c: case 0n 0n 0n: {==} case 0n 0n 1n+r: {==} case 0n 1n+q 0n: {==} case 0n 1n+q 1n+r: {==} case 1n+p 0n _: {==} case 1n+p 1n+q 0n: internal_and_false_sym(Nat.is_le(p, q)) case 1n+p 1n+q 1n+r: le_min(p, q, r)# The maximum is at most c exactly when both arguments are: max a b <= c tests as a <= c and b <= c.law max_le: for a: Nat for b: Nat for c: Nat {Nat.is_le(Nat.max(a, b), c) == Bool.and(Nat.is_le(a, c), Nat.is_le(b, c)) : Bool}def max_le(a, b, c): match a b c: case 0n _ 0n: {==} case 0n _ 1n+r: {==} case 1n+p 0n 0n: {==} case 1n+p 0n 1n+r: internal_and_true_sym(Nat.is_le(p, r)) case 1n+p 1n+q 0n: {==} case 1n+p 1n+q 1n+r: max_le(p, q, r)# Multiplying on the left preserves the order: a <= b implies k * a <= k * b.law mul_le_mul_left: for a: Nat for b: Nat for k: Nat for h: le(a, b) le(Nat.mul(k, a), Nat.mul(k, b))def mul_le_mul_left(a, b, k, h): +a = a +b = b +k = k %mul_comm(a, k) : le(_, Nat.mul(k, b)) %mul_comm(b, k) : le(Nat.mul(a, k), _) mul_le_mul_right(a, b, k, h)# Multiplying two bounded factors stays bounded: a <= c and b <= d imply a * b <= c * d.law mul_le_mul: for a: Nat for b: Nat for c: Nat for d: Nat for h1: le(a, c) for h2: le(b, d) le(Nat.mul(a, b), Nat.mul(c, d))def mul_le_mul(a, b, c, d, h1, h2): +a = a +b = b +c = c +d = d le_trans(Nat.mul(a, b), Nat.mul(c, b), Nat.mul(c, d), mul_le_mul_right(a, c, b, h1), mul_le_mul_left(b, d, c, h2))# --- generated: _sym twins (tools/mathlib/twins.ts), do not edit ---# Zero is a right identity for addition: x + 0 = x, reversed to rewrite toward the simple side.law add_zero_sym: for x: Nat {x == Nat.add(x, 0n) : Nat}def add_zero_sym(x): Equal.sym(Nat, Nat.add(x, 0n), x, add_zero(x))# Zero is a left identity for addition: 0 + x = x, reversed to rewrite toward the simple side.law zero_add_sym: for -x: Nat {x == Nat.add(0n, x) : Nat}def zero_add_sym(x): Equal.sym(Nat, Nat.add(0n, x), x, zero_add(x))# Adding a successor on the right: n + (m + 1) = (n + m) + 1, reversed to rewrite toward the simple side.law add_succ_sym: for n: Nat for -m: Nat {1n+Nat.add(n, m) == Nat.add(n, 1n+m) : Nat}def add_succ_sym(n, m): Equal.sym(Nat, Nat.add(n, 1n+m), 1n+Nat.add(n, m), add_succ(n, m))# Adding a successor on the left: (n + 1) + m = (n + m) + 1, reversed to rewrite toward the simple side.law succ_add_sym: for -n: Nat for -m: Nat {1n+Nat.add(n, m) == Nat.add(1n+n, m) : Nat}def succ_add_sym(n, m): Equal.sym(Nat, Nat.add(1n+n, m), 1n+Nat.add(n, m), succ_add(n, m))# Addition is commutative: n + m = m + n, reversed to rewrite toward the simple side.law add_comm_sym: for n: Nat for m: Nat {Nat.add(m, n) == Nat.add(n, m) : Nat}def add_comm_sym(n, m): Equal.sym(Nat, Nat.add(n, m), Nat.add(m, n), add_comm(n, m))# Addition is associative: (a + b) + c = a + (b + c), reversed to rewrite toward the simple side.law add_assoc_sym: for a: Nat for -b: Nat for -c: Nat {Nat.add(a, Nat.add(b, c)) == Nat.add(Nat.add(a, b), c) : Nat}def add_assoc_sym(a, b, c): Equal.sym(Nat, Nat.add(Nat.add(a, b), c), Nat.add(a, Nat.add(b, c)), add_assoc(a, b, c))# Left commutativity of addition: a + (b + c) = b + (a + c), reversed to rewrite toward the simple side.law add_left_comm_sym: for a: Nat for b: Nat for -c: Nat {Nat.add(b, Nat.add(a, c)) == Nat.add(a, Nat.add(b, c)) : Nat}def add_left_comm_sym(a, b, c): Equal.sym(Nat, Nat.add(a, Nat.add(b, c)), Nat.add(b, Nat.add(a, c)), add_left_comm(a, b, c))# Right commutativity of addition: (a + b) + c = (a + c) + b, reversed to rewrite toward the simple side.law add_right_comm_sym: for a: Nat for b: Nat for c: Nat {Nat.add(Nat.add(a, c), b) == Nat.add(Nat.add(a, b), c) : Nat}def add_right_comm_sym(a, b, c): Equal.sym(Nat, Nat.add(Nat.add(a, b), c), Nat.add(Nat.add(a, c), b), add_right_comm(a, b, c))# Four-way regrouping of a sum: (a + b) + (c + d) = (a + c) + (b + d), reversed to rewrite toward the simple side.law add_add_add_comm_sym: for a: Nat for b: Nat for c: Nat for -d: Nat {Nat.add(Nat.add(a, c), Nat.add(b, d)) == Nat.add(Nat.add(a, b), Nat.add(c, d)) : Nat}def add_add_add_comm_sym(a, b, c, d): Equal.sym(Nat, Nat.add(Nat.add(a, b), Nat.add(c, d)), Nat.add(Nat.add(a, c), Nat.add(b, d)), add_add_add_comm(a, b, c, d))# Zero absorbs multiplication on the right: x * 0 = 0, reversed to rewrite toward the simple side.law mul_zero_sym: for x: Nat {0n == Nat.mul(x, 0n) : Nat}def mul_zero_sym(x): Equal.sym(Nat, Nat.mul(x, 0n), 0n, mul_zero(x))# Zero absorbs multiplication on the left: 0 * x = 0, reversed to rewrite toward the simple side.law zero_mul_sym: for -x: Nat {0n == Nat.mul(0n, x) : Nat}def zero_mul_sym(x): Equal.sym(Nat, Nat.mul(0n, x), 0n, zero_mul(x))# One is a right identity for multiplication: x * 1 = x, reversed to rewrite toward the simple side.law mul_one_sym: for x: Nat {x == Nat.mul(x, 1n) : Nat}def mul_one_sym(x): Equal.sym(Nat, Nat.mul(x, 1n), x, mul_one(x))# One is a left identity for multiplication: 1 * x = x, reversed to rewrite toward the simple side.law one_mul_sym: for x: Nat {x == Nat.mul(1n, x) : Nat}def one_mul_sym(x): Equal.sym(Nat, Nat.mul(1n, x), x, one_mul(x))# Multiplying by a successor on the right: n * (m + 1) = n * m + n, reversed to rewrite toward the simple side.law mul_succ_sym: for n: Nat for m: Nat {Nat.add(Nat.mul(n, m), n) == Nat.mul(n, 1n+m) : Nat}def mul_succ_sym(n, m): Equal.sym(Nat, Nat.mul(n, 1n+m), Nat.add(Nat.mul(n, m), n), mul_succ(n, m))# Multiplying by a successor on the left: (n + 1) * m = n * m + m, reversed to rewrite toward the simple side.law succ_mul_sym: for n: Nat for m: Nat {Nat.add(Nat.mul(n, m), m) == Nat.mul(1n+n, m) : Nat}def succ_mul_sym(n, m): Equal.sym(Nat, Nat.mul(1n+n, m), Nat.add(Nat.mul(n, m), m), succ_mul(n, m))# Multiplication is commutative: n * m = m * n, reversed to rewrite toward the simple side.law mul_comm_sym: for n: Nat for m: Nat {Nat.mul(m, n) == Nat.mul(n, m) : Nat}def mul_comm_sym(n, m): Equal.sym(Nat, Nat.mul(n, m), Nat.mul(m, n), mul_comm(n, m))# Multiplication distributes over addition on the right: (a + b) * c = a * c + b * c, reversed to rewrite toward the simple side.law add_mul_sym: for a: Nat for -b: Nat for c: Nat {Nat.add(Nat.mul(a, c), Nat.mul(b, c)) == Nat.mul(Nat.add(a, b), c) : Nat}def add_mul_sym(a, b, c): Equal.sym(Nat, Nat.mul(Nat.add(a, b), c), Nat.add(Nat.mul(a, c), Nat.mul(b, c)), add_mul(a, b, c))# Multiplication distributes over addition on the left: a * (b + c) = a * b + a * c, reversed to rewrite toward the simple side.law mul_add_sym: for a: Nat for b: Nat for c: Nat {Nat.add(Nat.mul(a, b), Nat.mul(a, c)) == Nat.mul(a, Nat.add(b, c)) : Nat}def mul_add_sym(a, b, c): Equal.sym(Nat, Nat.mul(a, Nat.add(b, c)), Nat.add(Nat.mul(a, b), Nat.mul(a, c)), mul_add(a, b, c))# Multiplication is associative: (a * b) * c = a * (b * c), reversed to rewrite toward the simple side.law mul_assoc_sym: for a: Nat for b: Nat for c: Nat {Nat.mul(a, Nat.mul(b, c)) == Nat.mul(Nat.mul(a, b), c) : Nat}def mul_assoc_sym(a, b, c): Equal.sym(Nat, Nat.mul(Nat.mul(a, b), c), Nat.mul(a, Nat.mul(b, c)), mul_assoc(a, b, c))# Subtracting zero changes nothing: n - 0 = n, reversed to rewrite toward the simple side.law sub_zero_sym: for n: Nat {n == Nat.sub(n, 0n) : Nat}def sub_zero_sym(n): Equal.sym(Nat, Nat.sub(n, 0n), n, sub_zero(n))# Truncated subtraction from zero is zero: 0 - n = 0, reversed to rewrite toward the simple side.law zero_sub_sym: for n: Nat {0n == Nat.sub(0n, n) : Nat}def zero_sub_sym(n): Equal.sym(Nat, Nat.sub(0n, n), 0n, zero_sub(n))# A natural minus itself is zero: n - n = 0, reversed to rewrite toward the simple side.law sub_self_sym: for n: Nat {0n == Nat.sub(n, n) : Nat}def sub_self_sym(n): Equal.sym(Nat, Nat.sub(n, n), 0n, sub_self(n))# Subtracting successors: (n + 1) - (m + 1) = n - m, reversed to rewrite toward the simple side.law succ_sub_succ_sym: for -n: Nat for -m: Nat {Nat.sub(n, m) == Nat.sub(1n+n, 1n+m) : Nat}def succ_sub_succ_sym(n, m): Equal.sym(Nat, Nat.sub(1n+n, 1n+m), Nat.sub(n, m), succ_sub_succ(n, m))# Adding then subtracting m cancels: (n + m) - m = n, reversed to rewrite toward the simple side.law add_sub_cancel_sym: for n: Nat for m: Nat {n == Nat.sub(Nat.add(n, m), m) : Nat}def add_sub_cancel_sym(n, m): Equal.sym(Nat, Nat.sub(Nat.add(n, m), m), n, add_sub_cancel(n, m))# Adding then subtracting n cancels: (n + m) - n = m, reversed to rewrite toward the simple side.law add_sub_cancel_left_sym: for n: Nat for m: Nat {m == Nat.sub(Nat.add(n, m), n) : Nat}def add_sub_cancel_left_sym(n, m): Equal.sym(Nat, Nat.sub(Nat.add(n, m), n), m, add_sub_cancel_left(n, m))# Subtracting twice is subtracting the sum: (n - m) - k = n - (m + k), reversed to rewrite toward the simple side.law sub_sub_sym: for n: Nat for m: Nat for k: Nat {Nat.sub(n, Nat.add(m, k)) == Nat.sub(Nat.sub(n, m), k) : Nat}def sub_sub_sym(n, m, k): Equal.sym(Nat, Nat.sub(Nat.sub(n, m), k), Nat.sub(n, Nat.add(m, k)), sub_sub(n, m, k))# The division equation, for a positive divisor 1 + b: (a / (1 + b)) * (1 + b) + a % (1 + b) = a, reversed to rewrite toward the simple side.law div_mod_eq_sym: for +a: Nat for +b: Nat {a == Nat.add(Nat.mul(Nat.div(a, 1n+b), 1n+b), Nat.mod(a, 1n+b)) : Nat}def div_mod_eq_sym(a, b): Equal.sym(Nat, Nat.add(Nat.mul(Nat.div(a, 1n+b), 1n+b), Nat.mod(a, 1n+b)), a, div_mod_eq(a, b))# Adding the divisor adds one to the quotient: ((1 + b) + a) / (1 + b) = 1 + a / (1 + b), reversed to rewrite toward the simple side.law add_div_left_sym: for +a: Nat for +b: Nat {1n+Nat.div(a, 1n+b) == Nat.div(Nat.add(1n+b, a), 1n+b) : Nat}def add_div_left_sym(a, b): Equal.sym(Nat, Nat.div(Nat.add(1n+b, a), 1n+b), 1n+Nat.div(a, 1n+b), add_div_left(a, b))# A quotient is compared by multiplying back: n <= a / (1 + b) tests as n * (1 + b) <= a, reversed to rewrite toward the simple side.law le_div_iff_mul_le_sym: for n: Nat for +a: Nat for +b: Nat {Nat.is_le(Nat.mul(n, 1n+b), a) == Nat.is_le(n, Nat.div(a, 1n+b)) : Bool}def le_div_iff_mul_le_sym(n, a, b): Equal.sym(Bool, Nat.is_le(n, Nat.div(a, 1n+b)), Nat.is_le(Nat.mul(n, 1n+b), a), le_div_iff_mul_le(n, a, b))# Minimum is commutative, reversed to rewrite toward the simple side.law min_comm_sym: for a: Nat for b: Nat {Nat.min(b, a) == Nat.min(a, b) : Nat}def min_comm_sym(a, b): Equal.sym(Nat, Nat.min(a, b), Nat.min(b, a), min_comm(a, b))# Maximum is commutative, reversed to rewrite toward the simple side.law max_comm_sym: for a: Nat for b: Nat {Nat.max(b, a) == Nat.max(a, b) : Nat}def max_comm_sym(a, b): Equal.sym(Nat, Nat.max(a, b), Nat.max(b, a), max_comm(a, b))# The minimum of a natural and itself is itself, reversed to rewrite toward the simple side.law min_self_sym: for a: Nat {a == Nat.min(a, a) : Nat}def min_self_sym(a): Equal.sym(Nat, Nat.min(a, a), a, min_self(a))# The maximum of a natural and itself is itself, reversed to rewrite toward the simple side.law max_self_sym: for a: Nat {a == Nat.max(a, a) : Nat}def max_self_sym(a): Equal.sym(Nat, Nat.max(a, a), a, max_self(a))# The minimum with zero is zero: min(a, 0) = 0, reversed to rewrite toward the simple side.law min_zero_sym: for a: Nat {0n == Nat.min(a, 0n) : Nat}def min_zero_sym(a): Equal.sym(Nat, Nat.min(a, 0n), 0n, min_zero(a))# The minimum with zero is zero: min(0, a) = 0, reversed to rewrite toward the simple side.law zero_min_sym: for a: Nat {0n == Nat.min(0n, a) : Nat}def zero_min_sym(a): Equal.sym(Nat, Nat.min(0n, a), 0n, zero_min(a))# Zero is an identity for maximum: max(a, 0) = a, reversed to rewrite toward the simple side.law max_zero_sym: for a: Nat {a == Nat.max(a, 0n) : Nat}def max_zero_sym(a): Equal.sym(Nat, Nat.max(a, 0n), a, max_zero(a))# Zero is an identity for maximum: max(0, a) = a, reversed to rewrite toward the simple side.law zero_max_sym: for a: Nat {a == Nat.max(0n, a) : Nat}def zero_max_sym(a): Equal.sym(Nat, Nat.max(0n, a), a, zero_max(a))# Minimum is associative, reversed to rewrite toward the simple side.law min_assoc_sym: for a: Nat for b: Nat for c: Nat {Nat.min(a, Nat.min(b, c)) == Nat.min(Nat.min(a, b), c) : Nat}def min_assoc_sym(a, b, c): Equal.sym(Nat, Nat.min(Nat.min(a, b), c), Nat.min(a, Nat.min(b, c)), min_assoc(a, b, c))# Maximum is associative, reversed to rewrite toward the simple side.law max_assoc_sym: for a: Nat for b: Nat for c: Nat {Nat.max(a, Nat.max(b, c)) == Nat.max(Nat.max(a, b), c) : Nat}def max_assoc_sym(a, b, c): Equal.sym(Nat, Nat.max(Nat.max(a, b), c), Nat.max(a, Nat.max(b, c)), max_assoc(a, b, c))# The minimum plus the maximum is the sum: min(a, b) + max(a, b) = a + b, reversed to rewrite toward the simple side.law min_add_max_sym: for a: Nat for b: Nat {Nat.add(a, b) == Nat.add(Nat.min(a, b), Nat.max(a, b)) : Nat}def min_add_max_sym(a, b): Equal.sym(Nat, Nat.add(Nat.min(a, b), Nat.max(a, b)), Nat.add(a, b), min_add_max(a, b))# Any natural to the power zero is one, reversed to rewrite toward the simple side.law pow_zero_sym: for -a: Nat {1n == Nat.pow(a, 0n) : Nat}def pow_zero_sym(a): Equal.sym(Nat, Nat.pow(a, 0n), 1n, pow_zero(a))# A power with a successor exponent: a^(n+1) = a * a^n, reversed to rewrite toward the simple side.law pow_succ_sym: for -a: Nat for -n: Nat {Nat.mul(a, Nat.pow(a, n)) == Nat.pow(a, 1n+n) : Nat}def pow_succ_sym(a, n): Equal.sym(Nat, Nat.pow(a, 1n+n), Nat.mul(a, Nat.pow(a, n)), pow_succ(a, n))# Any natural to the power one is itself, reversed to rewrite toward the simple side.law pow_one_sym: for a: Nat {a == Nat.pow(a, 1n) : Nat}def pow_one_sym(a): Equal.sym(Nat, Nat.pow(a, 1n), a, pow_one(a))# One to any power is one, reversed to rewrite toward the simple side.law one_pow_sym: for n: Nat {1n == Nat.pow(1n, n) : Nat}def one_pow_sym(n): Equal.sym(Nat, Nat.pow(1n, n), 1n, one_pow(n))# Exponents add under multiplication: a^(m+n) = a^m * a^n, reversed to rewrite toward the simple side.law pow_add_sym: for a: Nat for m: Nat for n: Nat {Nat.mul(Nat.pow(a, m), Nat.pow(a, n)) == Nat.pow(a, Nat.add(m, n)) : Nat}def pow_add_sym(a, m, n): Equal.sym(Nat, Nat.pow(a, Nat.add(m, n)), Nat.mul(Nat.pow(a, m), Nat.pow(a, n)), pow_add(a, m, n))# Doubling is adding a natural to itself, reversed to rewrite toward the simple side.law double_eq_add_sym: for n: Nat {Nat.add(n, n) == Nat.double(n) : Nat}def double_eq_add_sym(n): Equal.sym(Nat, Nat.double(n), Nat.add(n, n), double_eq_add(n))# Every natural tests equal to itself, reversed to rewrite toward the simple side.law is_eq_refl_sym: for n: Nat {True{} == Nat.is_eq(n, n) : Bool}def is_eq_refl_sym(n): Equal.sym(Bool, Nat.is_eq(n, n), True{}, is_eq_refl(n))# The equality test is symmetric, reversed to rewrite toward the simple side.law is_eq_comm_sym: for a: Nat for b: Nat {Nat.is_eq(b, a) == Nat.is_eq(a, b) : Bool}def is_eq_comm_sym(a, b): Equal.sym(Bool, Nat.is_eq(a, b), Nat.is_eq(b, a), is_eq_comm(a, b))# A >= b tests the same as b <= a, reversed to rewrite toward the simple side.law is_ge_eq_is_le_sym: for a: Nat for b: Nat {Nat.is_le(b, a) == Nat.is_ge(a, b) : Bool}def is_ge_eq_is_le_sym(a, b): Equal.sym(Bool, Nat.is_ge(a, b), Nat.is_le(b, a), is_ge_eq_is_le(a, b))# A > b tests the same as b < a, reversed to rewrite toward the simple side.law is_gt_eq_is_lt_sym: for a: Nat for b: Nat {Nat.is_lt(b, a) == Nat.is_gt(a, b) : Bool}def is_gt_eq_is_lt_sym(a, b): Equal.sym(Bool, Nat.is_gt(a, b), Nat.is_lt(b, a), is_gt_eq_is_lt(a, b))# A < b tests the same as a + 1 <= b, reversed to rewrite toward the simple side.law is_lt_eq_succ_le_sym: for a: Nat for b: Nat {Nat.is_le(1n+a, b) == Nat.is_lt(a, b) : Bool}def is_lt_eq_succ_le_sym(a, b): Equal.sym(Bool, Nat.is_lt(a, b), Nat.is_le(1n+a, b), is_lt_eq_succ_le(a, b))# Not (a <= b) tests the same as b < a, reversed to rewrite toward the simple side.law not_is_le_sym: for a: Nat for b: Nat {Nat.is_lt(b, a) == Bool.not(Nat.is_le(a, b)) : Bool}def not_is_le_sym(a, b): Equal.sym(Bool, Bool.not(Nat.is_le(a, b)), Nat.is_lt(b, a), not_is_le(a, b))# Not (a < b) tests the same as b <= a, reversed to rewrite toward the simple side.law not_is_lt_sym: for a: Nat for b: Nat {Nat.is_le(b, a) == Bool.not(Nat.is_lt(a, b)) : Bool}def not_is_lt_sym(a, b): Equal.sym(Bool, Bool.not(Nat.is_lt(a, b)), Nat.is_le(b, a), not_is_lt(a, b))# Adding the same amount on the left does not change the order test: a <= b tests as k + a <= k + b, reversed to rewrite toward the simple side.law add_le_add_iff_left_sym: for k: Nat for -a: Nat for -b: Nat {Nat.is_le(Nat.add(k, a), Nat.add(k, b)) == Nat.is_le(a, b) : Bool}def add_le_add_iff_left_sym(k, a, b): Equal.sym(Bool, Nat.is_le(a, b), Nat.is_le(Nat.add(k, a), Nat.add(k, b)), add_le_add_iff_left(k, a, b))# A sum is at most c exactly when a <= c and b <= c - a: a + b <= c tests as both, reversed to rewrite toward the simple side.law le_and_le_sub_iff_add_le_sym: for a: Nat for -b: Nat for c: Nat {Nat.is_le(Nat.add(a, b), c) == Bool.and(Nat.is_le(a, c), Nat.is_le(b, Nat.sub(c, a))) : Bool}def le_and_le_sub_iff_add_le_sym(a, b, c): Equal.sym(Bool, Bool.and(Nat.is_le(a, c), Nat.is_le(b, Nat.sub(c, a))), Nat.is_le(Nat.add(a, b), c), le_and_le_sub_iff_add_le(a, b, c))# The minimum is at most c exactly when one argument is: min a b <= c tests as a <= c or b <= c, reversed to rewrite toward the simple side.law min_le_iff_sym: for a: Nat for b: Nat for c: Nat {Bool.or(Nat.is_le(a, c), Nat.is_le(b, c)) == Nat.is_le(Nat.min(a, b), c) : Bool}def min_le_iff_sym(a, b, c): Equal.sym(Bool, Nat.is_le(Nat.min(a, b), c), Bool.or(Nat.is_le(a, c), Nat.is_le(b, c)), min_le_iff(a, b, c))# The maximum is above a exactly when one argument is: a < max b c tests as a < b or a < c, reversed to rewrite toward the simple side.law lt_max_iff_sym: for a: Nat for b: Nat for c: Nat {Bool.or(Nat.is_lt(a, b), Nat.is_lt(a, c)) == Nat.is_lt(a, Nat.max(b, c)) : Bool}def lt_max_iff_sym(a, b, c): Equal.sym(Bool, Nat.is_lt(a, Nat.max(b, c)), Bool.or(Nat.is_lt(a, b), Nat.is_lt(a, c)), lt_max_iff(a, b, c))# Comparing against a difference is comparing the sum: a < c - b tests as b + a < c, reversed to rewrite toward the simple side.law lt_sub_iff_add_lt_sym: for a: Nat for b: Nat for c: Nat {Nat.is_lt(Nat.add(b, a), c) == Nat.is_lt(a, Nat.sub(c, b)) : Bool}def lt_sub_iff_add_lt_sym(a, b, c): Equal.sym(Bool, Nat.is_lt(a, Nat.sub(c, b)), Nat.is_lt(Nat.add(b, a), c), lt_sub_iff_add_lt(a, b, c))# Zero divided by anything is zero: 0 / b = 0, reversed to rewrite toward the simple side.law zero_div_sym: for b: Nat {0n == Nat.div(0n, b) : Nat}def zero_div_sym(b): Equal.sym(Nat, Nat.div(0n, b), 0n, zero_div(b))# Zero modulo anything is zero: 0 % b = 0, reversed to rewrite toward the simple side.law zero_mod_sym: for b: Nat {0n == Nat.mod(0n, b) : Nat}def zero_mod_sym(b): Equal.sym(Nat, Nat.mod(0n, b), 0n, zero_mod(b))# Dividing by one changes nothing: a / 1 = a, reversed to rewrite toward the simple side.law div_one_sym: for a: Nat {a == Nat.div(a, 1n) : Nat}def div_one_sym(a): Equal.sym(Nat, Nat.div(a, 1n), a, div_one(a))# Any natural modulo one is zero: a % 1 = 0, reversed to rewrite toward the simple side.law mod_one_sym: for a: Nat {0n == Nat.mod(a, 1n) : Nat}def mod_one_sym(a): Equal.sym(Nat, Nat.mod(a, 1n), 0n, mod_one(a))# A natural modulo itself is zero: n % n = 0, reversed to rewrite toward the simple side.law mod_self_sym: for n: Nat {0n == Nat.mod(n, n) : Nat}def mod_self_sym(n): Equal.sym(Nat, Nat.mod(n, n), 0n, mod_self(n))# A positive natural divided by itself is one: (1 + b) / (1 + b) = 1, reversed to rewrite toward the simple side.law div_self_sym: for b: Nat {1n == Nat.div(1n+b, 1n+b) : Nat}def div_self_sym(b): Equal.sym(Nat, Nat.div(1n+b, 1n+b), 1n, div_self(b))# Taking a remainder twice is taking it once: (a % n) % n = a % n, reversed to rewrite toward the simple side.law mod_mod_sym: for a: Nat for n: Nat {Nat.mod(a, n) == Nat.mod(Nat.mod(a, n), n) : Nat}def mod_mod_sym(a, n): Equal.sym(Nat, Nat.mod(Nat.mod(a, n), n), Nat.mod(a, n), mod_mod(a, n))# Adding the divisor does not change the remainder: (b + a) % b = a % b, reversed to rewrite toward the simple side.law add_mod_left_sym: for a: Nat for b: Nat {Nat.mod(a, b) == Nat.mod(Nat.add(b, a), b) : Nat}def add_mod_left_sym(a, b): Equal.sym(Nat, Nat.mod(Nat.add(b, a), b), Nat.mod(a, b), add_mod_left(a, b))# Multiplying by a positive divisor then dividing by it cancels: (a * (1 + b)) / (1 + b) = a, reversed to rewrite toward the simple side.law mul_div_cancel_sym: for a: Nat for b: Nat {a == Nat.div(Nat.mul(a, 1n+b), 1n+b) : Nat}def mul_div_cancel_sym(a, b): Equal.sym(Nat, Nat.div(Nat.mul(a, 1n+b), 1n+b), a, mul_div_cancel(a, b))# Multiplying on the left by a positive divisor then dividing by it cancels: ((1 + b) * a) / (1 + b) = a, reversed to rewrite toward the simple side.law mul_div_cancel_left_sym: for a: Nat for b: Nat {a == Nat.div(Nat.mul(1n+b, a), 1n+b) : Nat}def mul_div_cancel_left_sym(a, b): Equal.sym(Nat, Nat.div(Nat.mul(1n+b, a), 1n+b), a, mul_div_cancel_left(a, b))# A multiple of b leaves no remainder modulo b: (a * b) % b = 0, reversed to rewrite toward the simple side.law mul_mod_left_sym: for a: Nat for b: Nat {0n == Nat.mod(Nat.mul(a, b), b) : Nat}def mul_mod_left_sym(a, b): Equal.sym(Nat, Nat.mod(Nat.mul(a, b), b), 0n, mul_mod_left(a, b))# A multiple of a leaves no remainder modulo a: (a * b) % a = 0, reversed to rewrite toward the simple side.law mul_mod_right_sym: for a: Nat for b: Nat {0n == Nat.mod(Nat.mul(a, b), a) : Nat}def mul_mod_right_sym(a, b): Equal.sym(Nat, Nat.mod(Nat.mul(a, b), a), 0n, mul_mod_right(a, b))# The remainder plus the divisor times the quotient is the dividend: a % b + b * (a / b) = a, reversed to rewrite toward the simple side.law mod_add_div_sym: for a: Nat for b: Nat {a == Nat.add(Nat.mod(a, b), Nat.mul(b, Nat.div(a, b))) : Nat}def mod_add_div_sym(a, b): Equal.sym(Nat, Nat.add(Nat.mod(a, b), Nat.mul(b, Nat.div(a, b))), a, mod_add_div(a, b))# Left commutativity of multiplication: a * (b * c) = b * (a * c), reversed to rewrite toward the simple side.law mul_left_comm_sym: for a: Nat for b: Nat for c: Nat {Nat.mul(b, Nat.mul(a, c)) == Nat.mul(a, Nat.mul(b, c)) : Nat}def mul_left_comm_sym(a, b, c): Equal.sym(Nat, Nat.mul(a, Nat.mul(b, c)), Nat.mul(b, Nat.mul(a, c)), mul_left_comm(a, b, c))# Right commutativity of multiplication: (a * b) * c = (a * c) * b, reversed to rewrite toward the simple side.law mul_right_comm_sym: for a: Nat for b: Nat for c: Nat {Nat.mul(Nat.mul(a, c), b) == Nat.mul(Nat.mul(a, b), c) : Nat}def mul_right_comm_sym(a, b, c): Equal.sym(Nat, Nat.mul(Nat.mul(a, b), c), Nat.mul(Nat.mul(a, c), b), mul_right_comm(a, b, c))# Four-way regrouping of a product: (a * b) * (c * d) = (a * c) * (b * d), reversed to rewrite toward the simple side.law mul_mul_mul_comm_sym: for a: Nat for b: Nat for c: Nat for d: Nat {Nat.mul(Nat.mul(a, c), Nat.mul(b, d)) == Nat.mul(Nat.mul(a, b), Nat.mul(c, d)) : Nat}def mul_mul_mul_comm_sym(a, b, c, d): Equal.sym(Nat, Nat.mul(Nat.mul(a, b), Nat.mul(c, d)), Nat.mul(Nat.mul(a, c), Nat.mul(b, d)), mul_mul_mul_comm(a, b, c, d))# Zero to a positive power is zero: 0^(n+1) = 0, reversed to rewrite toward the simple side.law zero_pow_sym: for -n: Nat {0n == Nat.pow(0n, 1n+n) : Nat}def zero_pow_sym(n): Equal.sym(Nat, Nat.pow(0n, 1n+n), 0n, zero_pow(n))# A power of a product is the product of the powers: (a * b)^n = a^n * b^n, reversed to rewrite toward the simple side.law mul_pow_sym: for +a: Nat for +b: Nat for n: Nat {Nat.mul(Nat.pow(a, n), Nat.pow(b, n)) == Nat.pow(Nat.mul(a, b), n) : Nat}def mul_pow_sym(a, b, n): Equal.sym(Nat, Nat.pow(Nat.mul(a, b), n), Nat.mul(Nat.pow(a, n), Nat.pow(b, n)), mul_pow(a, b, n))# Exponents multiply under repeated powers: a^(m * n) = (a^m)^n, reversed to rewrite toward the simple side.law pow_mul_sym: for a: Nat for m: Nat for n: Nat {Nat.pow(Nat.pow(a, m), n) == Nat.pow(a, Nat.mul(m, n)) : Nat}def pow_mul_sym(a, m, n): Equal.sym(Nat, Nat.pow(a, Nat.mul(m, n)), Nat.pow(Nat.pow(a, m), n), pow_mul(a, m, n))# A common left summand cancels in a difference: (k + n) - (k + m) = n - m, reversed to rewrite toward the simple side.law add_sub_add_left_sym: for k: Nat for -n: Nat for -m: Nat {Nat.sub(n, m) == Nat.sub(Nat.add(k, n), Nat.add(k, m)) : Nat}def add_sub_add_left_sym(k, n, m): Equal.sym(Nat, Nat.sub(Nat.add(k, n), Nat.add(k, m)), Nat.sub(n, m), add_sub_add_left(k, n, m))# A common right summand cancels in a difference: (n + k) - (m + k) = n - m, reversed to rewrite toward the simple side.law add_sub_add_right_sym: for n: Nat for k: Nat for m: Nat {Nat.sub(n, m) == Nat.sub(Nat.add(n, k), Nat.add(m, k)) : Nat}def add_sub_add_right_sym(n, k, m): Equal.sym(Nat, Nat.sub(Nat.add(n, k), Nat.add(m, k)), Nat.sub(n, m), add_sub_add_right(n, k, m))# Multiplication distributes over subtraction on the right: (n - m) * k = n * k - m * k, reversed to rewrite toward the simple side.law sub_mul_sym: for n: Nat for m: Nat for k: Nat {Nat.sub(Nat.mul(n, k), Nat.mul(m, k)) == Nat.mul(Nat.sub(n, m), k) : Nat}def sub_mul_sym(n, m, k): Equal.sym(Nat, Nat.mul(Nat.sub(n, m), k), Nat.sub(Nat.mul(n, k), Nat.mul(m, k)), sub_mul(n, m, k))# Multiplication distributes over subtraction on the left: n * (m - k) = n * m - n * k, reversed to rewrite toward the simple side.law mul_sub_sym: for n: Nat for m: Nat for k: Nat {Nat.sub(Nat.mul(n, m), Nat.mul(n, k)) == Nat.mul(n, Nat.sub(m, k)) : Nat}def mul_sub_sym(n, m, k): Equal.sym(Nat, Nat.mul(n, Nat.sub(m, k)), Nat.sub(Nat.mul(n, m), Nat.mul(n, k)), mul_sub(n, m, k))# A number is at most the minimum exactly when it is at most both: a <= min b c tests as a <= b and a <= c, reversed to rewrite toward the simple side.law le_min_sym: for a: Nat for b: Nat for c: Nat {Bool.and(Nat.is_le(a, b), Nat.is_le(a, c)) == Nat.is_le(a, Nat.min(b, c)) : Bool}def le_min_sym(a, b, c): Equal.sym(Bool, Nat.is_le(a, Nat.min(b, c)), Bool.and(Nat.is_le(a, b), Nat.is_le(a, c)), le_min(a, b, c))# The maximum is at most c exactly when both arguments are: max a b <= c tests as a <= c and b <= c, reversed to rewrite toward the simple side.law max_le_sym: for a: Nat for b: Nat for c: Nat {Bool.and(Nat.is_le(a, c), Nat.is_le(b, c)) == Nat.is_le(Nat.max(a, b), c) : Bool}def max_le_sym(a, b, c): Equal.sym(Bool, Nat.is_le(Nat.max(a, b), c), Bool.and(Nat.is_le(a, c), Nat.is_le(b, c)), max_le(a, b, c))