/-
Copyright (c) 2014 Floris van Doorn. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.

Module: data.nat.order
Authors: Floris van Doorn, Leonardo de Moura, Jeremy Avigad

The order relation on the natural numbers.
-/

import .basic algebra.ordered_ring
open eq.ops

namespace nat

/- lt and le -/

theorem le_of_lt_or_eq {m n : ℕ} (H : m < n ∨ m = n) : m ≤ n :=
or.elim H (take H1, le_of_lt H1) (take H1, H1 ▸ !le.refl)

theorem lt.by_cases {a b : ℕ} {P : Prop}
  (H1 : a < b → P) (H2 : a = b → P) (H3 : b < a → P) : P :=
or.elim !lt.trichotomy
  (assume H, H1 H)
  (assume H, or.elim H (assume H', H2 H') (assume H', H3 H'))

theorem lt_or_eq_of_le {m n : ℕ} (H : m ≤ n) : m < n ∨ m = n :=
lt.by_cases
  (assume H1 : m < n, or.inl H1)
  (assume H1 : m = n, or.inr H1)
  (assume H1 : m > n, absurd (lt_of_le_of_lt H H1) !lt.irrefl)

theorem le_iff_lt_or_eq (m n : ℕ) : m ≤ n ↔ m < n ∨ m = n :=
iff.intro lt_or_eq_of_le le_of_lt_or_eq

theorem lt_of_le_and_ne {m n : ℕ} (H1 : m ≤ n) (H2 : m ≠ n) : m < n :=
or.elim (lt_or_eq_of_le H1)
  (take H3 : m < n, H3)
  (take H3 : m = n, absurd H3 H2)

theorem lt_iff_le_and_ne (m n : ℕ) : m < n ↔ m ≤ n ∧ m ≠ n :=
iff.intro
  (take H, and.intro (le_of_lt H) (take H1, lt.irrefl _ (H1 ▸ H)))
  (take H, lt_of_le_and_ne (and.elim_left H) (and.elim_right H))

theorem le_add_right (n k : ℕ) : n ≤ n + k :=
induction_on k
  (calc n ≤ n        : le.refl n
     ...  = n + zero : add_zero)
  (λ k (ih : n ≤ n + k), calc
     n   ≤ succ (n + k) : le_succ_of_le ih
     ... = n + succ k   : add_succ)

theorem le_add_left (n m : ℕ): n ≤ m + n :=
!add.comm ▸ !le_add_right

theorem le.intro {n m k : ℕ} (h : n + k = m) : n ≤ m :=
h ▸ le_add_right n k

theorem le.elim {n m : ℕ} (h : n ≤ m) : ∃k, n + k = m :=
le.rec_on h
  (exists.intro 0 rfl)
  (λ m (h : n < m), lt.rec_on h
    (exists.intro 1 rfl)
    (λ b hlt (ih : ∃ (k : ℕ), n + k = b),
      obtain (k : ℕ) (h : n + k = b), from ih,
      exists.intro (succ k) (calc
        n + succ k = succ (n + k) : add_succ
               ... = succ b       : h)))

theorem le.total {m n : ℕ} : m ≤ n ∨ n ≤ m :=
lt.by_cases
  (assume H : m < n, or.inl (le_of_lt H))
  (assume H : m = n, or.inl (H ▸ !le.refl))
  (assume H : m > n, or.inr (le_of_lt H))

/- addition -/

theorem add_le_add_left {n m : ℕ} (H : n ≤ m) (k : ℕ) : k + n ≤ k + m :=
obtain (l : ℕ) (Hl : n + l = m), from le.elim H,
le.intro
  (calc
      k + n + l  = k + (n + l) : !add.assoc
             ... = k + m       : {Hl})

theorem add_le_add_right {n m : ℕ} (H : n ≤ m) (k : ℕ) : n + k ≤ m + k :=
!add.comm ▸ !add.comm ▸ add_le_add_left H k

theorem le_of_add_le_add_left {k n m : ℕ} (H : k + n ≤ k + m) : n ≤ m :=
obtain (l : ℕ) (Hl : k + n + l = k + m), from (le.elim H),
le.intro (add.cancel_left
  (calc
      k + (n + l)  = k + n + l : (!add.assoc)⁻¹
               ... = k + m     : Hl))

theorem add_lt_add_left {n m : ℕ} (H : n < m) (k : ℕ) : k + n < k + m :=
lt_of_succ_le (!add_succ ▸ add_le_add_left (succ_le_of_lt H) k)

theorem add_lt_add_right {n m : ℕ} (H : n < m) (k : ℕ) : n + k < m + k :=
!add.comm ▸ !add.comm ▸ add_lt_add_left H k

theorem lt_add_of_pos_right {n k : ℕ} (H : k > 0) : n < n + k :=
!add_zero ▸ add_lt_add_left H n

/- multiplication -/

theorem mul_le_mul_left {n m : ℕ} (H : n ≤ m) (k : ℕ) : k * n ≤ k * m :=
obtain (l : ℕ) (Hl : n + l = m), from le.elim H,
have H2 : k * n + k * l = k * m, from
  calc
    k * n + k * l = k * (n + l) : mul.left_distrib
              ... = k * m       : {Hl},
le.intro H2

theorem mul_le_mul_right {n m : ℕ} (H : n ≤ m) (k : ℕ) : n * k ≤ m * k :=
!mul.comm ▸ !mul.comm ▸ (mul_le_mul_left H k)

theorem mul_le_mul {n m k l : ℕ} (H1 : n ≤ k) (H2 : m ≤ l) : n * m ≤ k * l :=
le.trans (mul_le_mul_right H1 m) (mul_le_mul_left H2 k)

theorem mul_lt_mul_of_pos_left {n m k : ℕ} (H : n < m) (Hk : k > 0) : k * n < k * m :=
have H2 : k * n < k * n + k, from lt_add_of_pos_right Hk,
have H3 : k * n + k ≤ k * m, from !mul_succ ▸ mul_le_mul_left (succ_le_of_lt H) k,
lt_of_lt_of_le H2 H3

theorem mul_lt_mul_of_pos_right {n m k : ℕ} (H : n < m) (Hk : k > 0) : n * k < m * k :=
!mul.comm ▸ !mul.comm ▸ mul_lt_mul_of_pos_left H Hk

theorem le.antisymm {n m : ℕ} (H1 : n ≤ m) (H2 : m ≤ n) : n = m :=
obtain (k : ℕ) (Hk : n + k = m), from (le.elim H1),
obtain (l : ℕ) (Hl : m + l = n), from (le.elim H2),
have L1 : k + l = 0, from
  add.cancel_left
    (calc
      n + (k + l) = n + k + l : (!add.assoc)⁻¹
        ... = m + l           : {Hk}
        ... = n               : Hl
        ... = n + 0           : (!add_zero)⁻¹),
have L2 : k = 0, from eq_zero_of_add_eq_zero_right L1,
calc
  n = n + 0  : (!add_zero)⁻¹
... = n  + k : {L2⁻¹}
... = m      : Hk

theorem zero_le (n : ℕ) : 0 ≤ n :=
le.intro !zero_add

/- nat is an instance of a linearly ordered semiring -/

section
  open [classes] algebra

  protected definition linear_ordered_semiring [instance] : algebra.linear_ordered_semiring nat :=
  algebra.linear_ordered_semiring.mk add add.assoc zero zero_add add_zero add.comm
    mul mul.assoc (succ zero) one_mul mul_one mul.left_distrib mul.right_distrib
    zero_mul mul_zero (ne.symm (succ_ne_zero zero)) @add.cancel_left @add.cancel_right le le.refl
    @le.trans @le.antisymm lt lt_iff_le_and_ne @add_le_add_left @le_of_add_le_add_left
    (take a b c H1 H2, mul_le_mul_left H1 c) (take a b c H1 H2, mul_le_mul_right H1 c)
    @mul_lt_mul_of_pos_left @mul_lt_mul_of_pos_right @le_iff_lt_or_eq @le.total
end

section port_algebra
  theorem ne_of_lt : ∀{a b : ℕ}, a < b → a ≠ b := @algebra.ne_of_lt _ _
  theorem lt_of_le_of_ne : ∀{a b : ℕ}, a ≤ b → a ≠ b → a < b :=
    @algebra.lt_of_le_of_ne _ _

  theorem not_le_of_lt : ∀{a b : ℕ}, a < b → ¬ b ≤ a := @algebra.not_le_of_lt _ _
  theorem not_lt_of_le : ∀{a b : ℕ}, a ≤ b → ¬ b < a := @algebra.not_lt_of_le _ _

  theorem le_of_not_lt : ∀{a b : ℕ}, ¬ a < b → b ≤ a := @algebra.le_of_not_lt _ _
  theorem lt_of_not_le : ∀{a b : ℕ}, ¬ a ≤ b → b < a := @algebra.lt_of_not_le _ _
  theorem lt_or_ge : ∀a b : ℕ, a < b ∨ a ≥ b := @algebra.lt_or_ge _ _
  theorem le_or_gt : ∀a b : ℕ, a ≤ b ∨ a > b := @algebra.le_or_gt _ _

  theorem add_le_add : ∀{a b c d : ℕ}, a ≤ b → c ≤ d → a + c ≤ b + d := @algebra.add_le_add _ _
  theorem add_lt_add : ∀{a b c d : ℕ}, a < b → c < d → a + c < b + d := @algebra.add_lt_add _ _
  theorem add_lt_add_of_le_of_lt : ∀{a b c d : ℕ}, a ≤ b → c < d → a + c < b + d :=
    @algebra.add_lt_add_of_le_of_lt _ _
  theorem add_lt_add_of_lt_of_le : ∀{a b c d : ℕ}, a < b → c ≤ d → a + c < b + d :=
    @algebra.add_lt_add_of_lt_of_le _ _
  theorem lt_add_of_pos_left : ∀{a b : ℕ}, b > 0 → a < b + a := @algebra.lt_add_of_pos_left _ _

  theorem le_of_add_le_add_right : ∀{a b c : ℕ}, a + b ≤ c + b → a ≤ c :=
    @algebra.le_of_add_le_add_right _ _
  theorem lt_of_add_lt_add_left : ∀{a b c : ℕ}, a + b < a + c → b < c :=
    @algebra.lt_of_add_lt_add_left _ _
  theorem lt_of_add_lt_add_right : ∀{a b c : ℕ}, a + b < c + b → a < c :=
    @algebra.lt_of_add_lt_add_right _ _
  theorem add_le_add_left_iff : ∀a b c : ℕ, a + b ≤ a + c ↔ b ≤ c := algebra.add_le_add_left_iff
  theorem add_le_add_right_iff : ∀a b c : ℕ, a + b ≤ c + b ↔ a ≤ c := algebra.add_le_add_right_iff
  theorem add_lt_add_left_iff : ∀a b c : ℕ, a + b < a + c ↔ b < c := algebra.add_lt_add_left_iff
  theorem add_lt_add_right_iff : ∀a b c : ℕ, a + b < c + b ↔ a < c := algebra.add_lt_add_right_iff

  theorem add_pos_left : ∀{a : ℕ}, 0 < a → ∀b : ℕ, 0 < a + b :=
    take a H b, @algebra.add_pos_of_pos_of_nonneg _ _ a b H !zero_le
  theorem add_pos_right : ∀{a : ℕ}, 0 < a → ∀b : ℕ, 0 < b + a :=
    take a H b, !add.comm ▸ add_pos_left H b

  theorem add_eq_zero_iff_eq_zero_and_eq_zero : ∀{a b : ℕ},
    a + b = 0 ↔ a = 0 ∧ b = 0 :=
    take a b : ℕ,
      @algebra.add_eq_zero_iff_eq_zero_and_eq_zero_of_nonneg_of_nonneg _ _ a b !zero_le !zero_le

  theorem le_add_of_le_left : ∀{a b c : ℕ}, b ≤ c → b ≤ a + c :=
    take a b c H, @algebra.le_add_of_nonneg_of_le _ _ a b c !zero_le H
  theorem le_add_of_le_right : ∀{a b c : ℕ}, b ≤ c → b ≤ c + a :=
    take a b c H, @algebra.le_add_of_le_of_nonneg _ _ a b c H !zero_le
  theorem lt_add_of_pos_of_le : ∀{a b c : ℕ}, 0 < a → b ≤ c → b < a + c :=
    @algebra.lt_add_of_pos_of_le _ _
  theorem lt_add_of_le_of_pos : ∀{a b c : ℕ}, b ≤ c → 0 < a → b < c + a :=
    @algebra.lt_add_of_le_of_pos _ _

  theorem lt_add_of_lt_left : ∀{b c : ℕ}, b < c → ∀a, b < a + c :=
    take b c H a, @algebra.lt_add_of_nonneg_of_lt _ _ a b c !zero_le H
  theorem lt_add_of_lt_right : ∀{b c : ℕ}, b < c → ∀a, b < c + a :=
    take b c H a, @algebra.lt_add_of_lt_of_nonneg _ _ a b c H !zero_le
  theorem lt_add_of_pos_of_lt : ∀{a b c : ℕ}, 0 < a → b < c → b < a + c :=
    @algebra.lt_add_of_pos_of_lt _ _
  theorem lt_add_of_lt_of_pos : ∀{a b c : ℕ}, b < c → 0 < a → b < c + a :=
    @algebra.lt_add_of_lt_of_pos _ _

  theorem mul_pos : ∀{a b : ℕ}, 0 < a → 0 < b → 0 < a * b := @algebra.mul_pos _ _

  theorem lt_of_mul_lt_mul_left : ∀{a b c : ℕ}, c * a < c * b → a < b :=
    take a b c H, @algebra.lt_of_mul_lt_mul_left _ _ a b c H !zero_le
  theorem lt_of_mul_lt_mul_right : ∀{a b c : ℕ}, a * c < b * c → a < b :=
    take a b c H, @algebra.lt_of_mul_lt_mul_right _ _ a b c H !zero_le
  theorem le_of_mul_le_mul_left : ∀{a b c : ℕ}, c * a ≤ c * b → c > 0 → a ≤ b :=
    @algebra.le_of_mul_le_mul_left _ _
  theorem le_of_mul_le_mul_right : ∀{a b c : ℕ}, a * c ≤ b * c → c > 0 → a ≤ b :=
    @algebra.le_of_mul_le_mul_right _ _
  theorem pos_of_mul_pos_left : ∀{a b : ℕ}, 0 < a * b → 0 < b :=
    take a b H, @algebra.pos_of_mul_pos_left _ _ a b H !zero_le
  theorem pos_of_mul_pos_right : ∀{a b : ℕ}, 0 < a * b → 0 < a :=
    take a b H, @algebra.pos_of_mul_pos_right _ _ a b H !zero_le
end port_algebra

theorem zero_le_one : 0 ≤ 1 := dec_trivial
theorem zero_lt_one : 0 < 1 := dec_trivial

/- properties specific to nat -/

theorem lt_intro {n m k : ℕ} (H : succ n + k = m) : n < m :=
lt_of_succ_le (le.intro H)

theorem lt_elim {n m : ℕ} (H : n < m) : ∃k, succ n + k = m :=
le.elim (succ_le_of_lt H)

theorem lt_add_succ (n m : ℕ) : n < n + succ m :=
lt_intro !succ_add_eq_add_succ

theorem eq_zero_of_le_zero {n : ℕ} (H : n ≤ 0) : n = 0 :=
obtain (k : ℕ) (Hk : n + k = 0), from le.elim H,
eq_zero_of_add_eq_zero_right Hk

/- succ and pred -/

theorem lt_iff_succ_le (m n : nat) : m < n ↔ succ m ≤ n :=
iff.intro succ_le_of_lt lt_of_succ_le

theorem not_succ_le_zero (n : ℕ) : ¬ succ n ≤ 0 :=
(assume H : succ n ≤ 0,
  have H2 : succ n = 0, from eq_zero_of_le_zero H,
  absurd H2 !succ_ne_zero)

theorem succ_le_succ {n m : ℕ} (H : n ≤ m) : succ n ≤ succ m :=
!add_one ▸ !add_one ▸ add_le_add_right H 1

theorem le_of_succ_le_succ {n m : ℕ} (H : succ n ≤ succ m) : n ≤ m :=
le_of_add_le_add_right ((!add_one)⁻¹ ▸ (!add_one)⁻¹ ▸ H)

theorem self_le_succ (n : ℕ) : n ≤ succ n :=
le.intro !add_one

theorem succ_le_or_eq_of_le {n m : ℕ} (H : n ≤ m) : succ n ≤ m ∨ n = m :=
or.elim (lt_or_eq_of_le H)
  (assume H1 : n < m, or.inl (succ_le_of_lt H1))
  (assume H1 : n = m, or.inr H1)

theorem le_succ_of_pred_le {n m : ℕ} : pred n ≤ m → n ≤ succ m :=
nat.cases_on n
  (assume H : pred 0 ≤ m, !zero_le)
  (take n',
    assume H : pred (succ n') ≤ m,
    have H1 : n' ≤ m, from pred_succ n' ▸ H,
    succ_le_succ H1)

theorem pred_le_of_le_succ {n m : ℕ} : n ≤ succ m → pred n ≤ m :=
nat.cases_on n
  (assume H, !pred_zero⁻¹ ▸ zero_le m)
  (take n',
    assume H : succ n' ≤ succ m,
    have H1 : n' ≤ m, from le_of_succ_le_succ H,
    !pred_succ⁻¹ ▸ H1)

theorem succ_le_of_le_pred {n m : ℕ} : succ n ≤ m → n ≤ pred m :=
nat.cases_on m
  (assume H, absurd H !not_succ_le_zero)
  (take m',
    assume H : succ n ≤ succ m',
    have H1 : n ≤ m', from le_of_succ_le_succ H,
    !pred_succ⁻¹ ▸ H1)

theorem pred_le_pred_of_le {n m : ℕ} : n ≤ m → pred n ≤ pred m :=
nat.cases_on n
  (assume H, pred_zero⁻¹ ▸ zero_le (pred m))
  (take n',
    assume H : succ n' ≤ m,
    !pred_succ⁻¹ ▸ succ_le_of_le_pred H)

theorem le_or_eq_succ_of_le_succ {n m : ℕ} (H : n ≤ succ m) : n ≤ m ∨ n = succ m :=
or_of_or_of_imp_left (succ_le_or_eq_of_le H)
   (take H2 : succ n ≤ succ m, show n ≤ m, from le_of_succ_le_succ H2)

theorem le_pred_self (n : ℕ) : pred n ≤ n :=
cases_on n
  (pred_zero⁻¹ ▸ !le.refl)
  (take k : ℕ, (!pred_succ)⁻¹ ▸ !self_le_succ)

theorem succ_pos (n : ℕ) : 0 < succ n :=
!zero_lt_succ

theorem succ_pred_of_pos {n : ℕ} (H : n > 0) : succ (pred n) = n :=
(or_resolve_right (eq_zero_or_eq_succ_pred n) (ne.symm (ne_of_lt H)))⁻¹

theorem exists_eq_succ_of_lt {n m : ℕ} (H : n < m) : exists k, m = succ k :=
discriminate
  (take (Hm : m = 0), absurd (Hm ▸ H) !not_lt_zero)
  (take (l : ℕ) (Hm : m = succ l), exists.intro l Hm)

theorem self_lt_succ (n : ℕ) : n < succ n :=
lt.base n

theorem le_of_lt_succ {n m : ℕ} (H : n < succ m) : n ≤ m :=
le_of_succ_le_succ (succ_le_of_lt H)

/- other forms of induction -/

protected theorem strong_induction_on {P : nat → Prop} (n : ℕ) (H : ∀n, (∀m, m < n → P m) → P n) :
    P n :=
have H1 : ∀ {n m : nat}, m < n → P m, from
  take n,
  induction_on n
    (show ∀m, m < 0 → P m, from take m H, absurd H !not_lt_zero)
    (take n',
      assume IH : ∀ {m : nat}, m < n' → P m,
      have H2: P n', from H n' @IH,
      show ∀m, m < succ n' → P m, from
        take m,
        assume H3 : m < succ n',
        or.elim (lt_or_eq_of_le (le_of_lt_succ H3))
          (assume H4: m < n', IH H4)
          (assume H4: m = n', H4⁻¹ ▸ H2)),
H1 !self_lt_succ

protected theorem case_strong_induction_on {P : nat → Prop} (a : nat) (H0 : P 0)
  (Hind : ∀(n : nat), (∀m, m ≤ n → P m) → P (succ n)) : P a :=
strong_induction_on a (
  take n,
  show (∀m, m < n → P m) → P n, from
    cases_on n
       (assume H : (∀m, m < 0 → P m), show P 0, from H0)
       (take n,
         assume H : (∀m, m < succ n → P m),
         show P (succ n), from
           Hind n (take m, assume H1 : m ≤ n, H _ (lt_succ_of_le H1))))

/- pos -/

theorem by_cases_zero_pos {P : ℕ → Prop} (y : ℕ) (H0 : P 0) (H1 : ∀ {y : nat}, y > 0 → P y) : P y :=
cases_on y H0 (take y, H1 !succ_pos)

theorem zero_or_pos {n : ℕ} : n = 0 ∨ n > 0 :=
or_of_or_of_imp_left
  (or.swap (lt_or_eq_of_le !zero_le))
  (take H : 0 = n, H⁻¹)

theorem pos_of_ne_zero {n : ℕ} (H : n ≠ 0) : n > 0 :=
or.elim zero_or_pos (take H2 : n = 0, absurd H2 H) (take H2 : n > 0, H2)

theorem ne_zero_of_pos {n : ℕ} (H : n > 0) : n ≠ 0 :=
ne.symm (ne_of_lt H)

theorem exists_eq_succ_of_pos {n : ℕ} (H : n > 0) : exists l, n = succ l :=
exists_eq_succ_of_lt H

/- multiplication -/

theorem mul_lt_mul_of_le_of_lt {n m k l : ℕ} (Hk : k > 0) (H1 : n ≤ k) (H2 : m < l) :
  n * m < k * l :=
lt_of_le_of_lt (mul_le_mul_right H1 m) (mul_lt_mul_of_pos_left H2 Hk)

theorem mul_lt_mul_of_lt_of_le {n m k l : ℕ} (Hl : l > 0) (H1 : n < k) (H2 : m ≤ l) :
  n * m < k * l :=
lt_of_le_of_lt (mul_le_mul_left H2 n) (mul_lt_mul_of_pos_right H1 Hl)

theorem mul_lt_mul_of_le_of_le {n m k l : ℕ} (H1 : n < k) (H2 : m < l) : n * m < k * l :=
have H3 : n * m ≤ k * m, from mul_le_mul_right (le_of_lt H1) m,
have H4 : k * m < k * l, from mul_lt_mul_of_pos_left H2 (lt_of_le_of_lt !zero_le H1),
lt_of_le_of_lt H3 H4

theorem eq_of_mul_eq_mul_left {m k n : ℕ} (Hn : n > 0) (H : n * m = n * k) : m = k :=
have H2 : n * m ≤ n * k, from H ▸ !le.refl,
have H3 : n * k ≤ n * m, from H ▸ !le.refl,
have H4 : m ≤ k, from le_of_mul_le_mul_left H2 Hn,
have H5 : k ≤ m, from le_of_mul_le_mul_left H3 Hn,
le.antisymm H4 H5

theorem eq_zero_or_eq_of_mul_eq_mul_left {n m k : ℕ} (H : n * m = n * k) : n = 0 ∨ m = k :=
or_of_or_of_imp_right zero_or_pos
  (assume Hn : n > 0, eq_of_mul_eq_mul_left Hn H)

theorem eq_of_mul_eq_mul_right {n m k : ℕ} (Hm : m > 0) (H : n * m = k * m) : n = k :=
eq_of_mul_eq_mul_left Hm (!mul.comm ▸ !mul.comm ▸ H)

theorem eq_zero_or_eq_of_mul_eq_mul_right  {n m k : ℕ} (H : n * m = k * m) : m = 0 ∨ n = k :=
eq_zero_or_eq_of_mul_eq_mul_left (!mul.comm ▸ !mul.comm ▸ H)

theorem eq_one_of_mul_eq_one_left {n m : ℕ} (H : n * m = 1) : n = 1 :=
have H2 : n * m > 0, from H⁻¹ ▸ !succ_pos,
have H3 : n > 0, from pos_of_mul_pos_right H2,
have H4 : m > 0, from pos_of_mul_pos_left H2,
or.elim (le_or_gt n 1)
  (assume H5 : n ≤ 1,
    show n = 1, from le.antisymm H5 (succ_le_of_lt H3))
  (assume H5 : n > 1,
    have H6 : n * m ≥ 2 * 1, from mul_le_mul (succ_le_of_lt H5) (succ_le_of_lt H4),
    have H7 : 1 ≥ 2, from !mul_one ▸ H ▸ H6,
    absurd !self_lt_succ (not_lt_of_le H7))

theorem eq_one_of_mul_eq_one_right {n m : ℕ} (H : n * m = 1) : m = 1 :=
eq_one_of_mul_eq_one_left (!mul.comm ▸ H)

end nat