----------------------------------------------------------------------------------------------------
-- Copyright (c) 2014 Floris van Doorn. All rights reserved.
-- Released under Apache 2.0 license as described in the file LICENSE.
-- Author: Floris van Doorn
----------------------------------------------------------------------------------------------------
import logic algebra.binary
open tactic binary eq_ops eq
open decidable

inductive nat : Type :=
zero : nat,
succ : nat → nat


namespace nat
notation `ℕ`:max := nat

definition plus (x y : ℕ) : ℕ
:= nat.rec x (λ n r, succ r) y

definition to_nat [coercion] (n : num) : ℕ
:= num.rec zero (λ n, pos_num.rec (succ zero) (λ n r, plus r (plus r (succ zero))) (λ n r, plus r r) n) n

namespace helper_tactics
  definition apply_refl := apply @eq.refl
  tactic_hint apply_refl
end helper_tactics
open helper_tactics

theorem nat_rec_zero {P : ℕ → Type} (x : P 0) (f : ∀m, P m → P (succ m)) : nat.rec x f 0 = x

theorem nat_rec_succ {P : ℕ → Type} (x : P 0) (f : ∀m, P m → P (succ m)) (n : ℕ) : nat.rec x f (succ n) = f n (nat.rec x f n)

theorem induction_on {P : ℕ → Prop} (a : ℕ) (H1 : P 0) (H2 : ∀ (n : ℕ) (IH : P n), P (succ n)) : P a
:= nat.rec H1 H2 a

definition rec_on {P : ℕ → Type} (n : ℕ) (H1 : P 0) (H2 : ∀m, P m → P (succ m)) : P n
:= nat.rec H1 H2 n

-------------------------------------------------- succ pred

theorem succ_ne_zero (n : ℕ) : succ n ≠ 0
:= assume H : succ n = 0,
     have H2 : true = false, from
     let f := (nat.rec false (fun a b, true)) in
       calc true = f (succ n) : _
             ... = f 0        : {H}
             ... = false      : _,
     absurd H2 true_ne_false

definition pred (n : ℕ) := nat.rec 0 (fun m x, m) n

theorem pred_zero : pred 0 = 0

theorem pred_succ (n : ℕ) : pred (succ n) = n

theorem zero_or_succ (n : ℕ) : n = 0 ∨ n = succ (pred n)
:= induction_on n
    (or.intro_left _ (eq.refl 0))
    (take m IH, or.intro_right _
      (show succ m = succ (pred (succ m)), from congr_arg succ (pred_succ m⁻¹)))

theorem zero_or_succ2 (n : ℕ) : n = 0 ∨ ∃k, n = succ k
:= or.imp_or (zero_or_succ n) (assume H, H) (assume H : n = succ (pred n), exists_intro (pred n) H)

theorem case {P : ℕ → Prop} (n : ℕ) (H1: P 0) (H2 : ∀m, P (succ m)) : P n
:= induction_on n H1 (take m IH, H2 m)

theorem discriminate {B : Prop} {n : ℕ} (H1: n = 0 → B) (H2 : ∀m, n = succ m → B) : B
:= or.elim (zero_or_succ n)
    (take H3 : n = 0, H1 H3)
    (take H3 : n = succ (pred n), H2 (pred n) H3)

theorem succ_inj {n m : ℕ} (H : succ n = succ m) : n = m
:= calc
    n = pred (succ n) : pred_succ n⁻¹
  ... = pred (succ m) : {H}
  ... = m             : pred_succ m

theorem succ_ne_self (n : ℕ) : succ n ≠ n
:= induction_on n
    (take H : 1 = 0,
      have ne : 1 ≠ 0, from succ_ne_zero 0,
      absurd H ne)
    (take k IH H, IH (succ_inj H))

theorem decidable_eq [instance] (n m : ℕ) : decidable (n = m)
:= have general : ∀n, decidable (n = m), from
     rec_on m
       (take n,
         rec_on n
           (inl (eq.refl 0))
           (λ m iH, inr (succ_ne_zero m)))
       (λ (m' : ℕ) (iH1 : ∀n, decidable (n = m')),
         take n, rec_on n
           (inr (ne.symm (succ_ne_zero m')))
           (λ (n' : ℕ) (iH2 : decidable (n' = succ m')),
             have d1 : decidable (n' = m'), from iH1 n',
             decidable.rec_on d1
               (assume Heq : n' = m', inl (congr_arg succ Heq))
               (assume Hne : n' ≠ m',
                 have H1 : succ n' ≠ succ m', from
                   assume Heq, absurd (succ_inj Heq) Hne,
                 inr H1))),
  general n

theorem two_step_induction_on {P : ℕ → Prop} (a : ℕ) (H1 : P 0) (H2 : P 1)
    (H3 : ∀ (n : ℕ) (IH1 : P n) (IH2 : P (succ n)), P (succ (succ n))) : P a
:= have stronger : P a ∧ P (succ a), from
    induction_on a
      (and.intro H1 H2)
      (take k IH,
        have IH1 : P k, from and.elim_left IH,
        have IH2 : P (succ k), from and.elim_right IH,
          and.intro IH2 (H3 k IH1 IH2)),
    and.elim_left stronger

theorem sub_induction {P : ℕ → ℕ → Prop} (n m : ℕ) (H1 : ∀m, P 0 m)
   (H2 : ∀n, P (succ n) 0) (H3 : ∀n m, P n m → P (succ n) (succ m)) : P n m
:= have general : ∀m, P n m, from induction_on n
    (take m : ℕ, H1 m)
    (take k : ℕ,
      assume IH : ∀m, P k m,
      take m : ℕ,
      discriminate
        (assume Hm : m = 0,
          Hm⁻¹ ▸ (H2 k))
        (take l : ℕ,
          assume Hm : m = succ l,
            Hm⁻¹ ▸ (H3 k l (IH l)))),
  general m

-------------------------------------------------- add
definition add (x y : ℕ) : ℕ := plus x y
infixl `+`:65 := add
theorem add_zero_right (n : ℕ) : n + 0 = n
theorem add_succ_right (n m : ℕ) : n + succ m = succ (n + m)
---------- comm, assoc

theorem add_zero_left (n : ℕ) : 0 + n = n
:= induction_on n
    (add_zero_right 0)
    (take m IH, show 0 + succ m = succ m, from
      calc
        0 + succ m = succ (0 + m) : add_succ_right _ _
            ... = succ m : {IH})

theorem add_succ_left (n m : ℕ) : (succ n) + m = succ (n + m)
:= induction_on m
    (calc
      succ n + 0 = succ n : add_zero_right (succ n)
        ... = succ (n + 0) : {symm (add_zero_right n)})
    (take k IH,
      calc
        succ n + succ k = succ (succ n + k) : add_succ_right _ _
          ... = succ (succ (n + k)) : {IH}
          ... = succ (n + succ k) : {symm (add_succ_right _ _)})

theorem add_comm (n m : ℕ) : n + m = m + n
:= induction_on m
    (trans (add_zero_right _) (symm (add_zero_left _)))
    (take k IH,
      calc
        n + succ k = succ (n+k) : add_succ_right _ _
          ... = succ (k + n) : {IH}
          ... = succ k + n : symm (add_succ_left _ _))

theorem add_move_succ (n m : ℕ) : succ n + m = n + succ m
:= calc
    succ n + m = succ (n + m) : add_succ_left n m
      ... = n +succ m : symm (add_succ_right n m)

theorem add_comm_succ (n m : ℕ) : n + succ m = m + succ n
:= calc
    n + succ m = succ n + m : symm (add_move_succ n m)
      ... = m + succ n : add_comm (succ n) m

theorem add_assoc (n m k : ℕ) : (n + m) + k = n + (m + k)
:= induction_on k
    (calc
      (n + m) + 0 = n + m : add_zero_right _
        ... = n + (m + 0) : {symm (add_zero_right m)})
    (take l IH,
      calc
        (n + m) + succ l = succ ((n + m) + l) : add_succ_right _ _
          ... = succ (n + (m + l)) : {IH}
          ... = n + succ (m + l) : symm (add_succ_right _ _)
          ... = n + (m + succ l) : {symm (add_succ_right _ _)})

theorem add_left_comm (n m k : ℕ) : n + (m + k) = m + (n + k)
:= left_comm add_comm add_assoc n m k

theorem add_right_comm (n m k : ℕ) : n + m + k = n + k + m
:= right_comm add_comm add_assoc n m k


---------- inversion

theorem add_cancel_left {n m k : ℕ} : n + m = n + k → m = k
:=
  induction_on n
    (take H : 0 + m = 0 + k,
      calc
        m = 0 + m : symm (add_zero_left m)
          ... = 0 + k : H
          ... = k : add_zero_left k)
    (take (n : ℕ) (IH : n + m = n + k → m = k) (H : succ n + m = succ n + k),
      have H2 : succ (n + m) = succ (n + k),
      from calc
        succ (n + m) = succ n + m : symm (add_succ_left n m)
          ... = succ n + k : H
          ... = succ (n + k) : add_succ_left n k,
      have H3 : n + m = n + k, from succ_inj H2,
      IH H3)

--rename to and_cancel_right
theorem add_cancel_right {n m k : ℕ} (H : n + m = k + m) : n = k
:=
  have H2 : m + n = m + k,
    from calc
      m + n = n + m : add_comm m n
      ... = k + m : H
      ... = m + k : add_comm k m,
    add_cancel_left H2

theorem add_eq_zero_left {n m : ℕ} : n + m = 0 → n = 0
:=
  induction_on n
    (take (H : 0 + m = 0), eq.refl 0)
    (take k IH,
      assume (H : succ k + m = 0),
      absurd
        (show succ (k + m) = 0, from
          calc
            succ (k + m) = succ k + m : symm (add_succ_left k m)
              ... = 0 : H)
        (succ_ne_zero (k + m)))

theorem add_eq_zero_right {n m : ℕ} (H : n + m = 0) : m = 0
:= add_eq_zero_left (trans (add_comm m n) H)

theorem add_eq_zero {n m : ℕ} (H : n + m = 0) : n = 0 ∧ m = 0
:= and.intro (add_eq_zero_left H) (add_eq_zero_right H)

-- add_eq_self below

---------- misc

theorem add_one (n:ℕ) : n + 1 = succ n
:=
  calc
    n + 1 = succ (n + 0) : add_succ_right _ _
      ... = succ n : {add_zero_right _}

theorem add_one_left (n:ℕ) : 1 + n = succ n
:=
  calc
    1 + n = succ (0 + n) : add_succ_left _ _
      ... = succ n : {add_zero_left _}

--the following theorem has a terrible name, but since the name is not a substring or superstring of another name, it is at least easy to globally replace it
theorem induction_plus_one {P : ℕ → Prop} (a : ℕ) (H1 : P 0)
    (H2 : ∀ (n : ℕ) (IH : P n), P (n + 1)) : P a
:= nat.rec H1 (take n IH, (add_one n) ▸ (H2 n IH)) a

-------------------------------------------------- mul

definition mul (n m : ℕ) := nat.rec 0 (fun m x, x + n) m
infixl `*`:75 := mul

theorem mul_zero_right (n:ℕ) : n * 0 = 0
theorem mul_succ_right (n m:ℕ) : n * succ m = n * m + n
set_option unifier.max_steps 100000
---------- comm, distr, assoc, identity

theorem mul_zero_left (n:ℕ) : 0 * n = 0
:= induction_on n
    (mul_zero_right 0)
    (take m IH,
      calc
        0 * succ m = 0 * m + 0 : mul_succ_right _ _
          ... = 0 * m : add_zero_right _
          ... = 0 : IH)

theorem mul_succ_left (n m:ℕ) : (succ n) * m = (n * m) + m
:= induction_on m
    (calc
      succ n * 0 = 0     : mul_zero_right _
        ... = n * 0      : symm (mul_zero_right _)
        ... = n * 0 + 0  : symm (add_zero_right _))
    (take k IH,
      calc
        succ n * succ k = (succ n * k) + succ n : mul_succ_right _ _
            ... = (n * k) + k + succ n : { IH }
            ... = (n * k) + (k + succ n) : add_assoc _ _ _
            ... = (n * k) + (n + succ k) : {add_comm_succ _ _}
            ... = (n * k) + n + succ k : symm (add_assoc _ _ _)
            ... = (n * succ k) + succ k : {symm (mul_succ_right n k)})

theorem mul_comm (n m:ℕ) : n * m = m * n
:= induction_on m
    (trans (mul_zero_right _) (symm (mul_zero_left _)))
    (take k IH,
      calc
        n * succ k = n * k + n : mul_succ_right _ _
          ... = k * n + n : {IH}
          ... = (succ k) * n : symm (mul_succ_left _ _))

theorem mul_add_distr_left (n m k : ℕ) : (n + m) * k = n * k + m * k
:= induction_on k
    (calc
      (n + m) * 0 = 0 : mul_zero_right _
        ... = 0 + 0 : symm (add_zero_right _)
        ... = n * 0 + 0 : eq.refl _
        ... = n * 0 + m * 0 : eq.refl _)
    (take l IH, calc
        (n + m) * succ l = (n + m) * l + (n + m) : mul_succ_right _ _
          ... = n * l + m * l + (n + m) : {IH}
          ... = n * l + m * l + n + m : symm (add_assoc _ _ _)
          ... = n * l + n + m * l + m : {add_right_comm _ _ _}
          ... = n * l + n + (m * l + m) : add_assoc _ _ _
          ... = n * succ l + (m * l + m) : {symm (mul_succ_right _ _)}
          ... = n * succ l + m * succ l : {symm (mul_succ_right _ _)})

theorem mul_add_distr_right (n m k : ℕ) : n * (m + k) = n * m + n * k
:= calc
    n * (m + k) = (m + k) * n : mul_comm _ _
      ... = m * n + k * n : mul_add_distr_left _ _ _
      ... = n * m + k * n : {mul_comm _ _}
      ... = n * m + n * k : {mul_comm _ _}

theorem mul_assoc (n m k:ℕ) : (n * m) * k = n * (m * k)
:= induction_on k
    (calc
      (n * m) * 0 = 0 : mul_zero_right _
        ... = n * 0 : symm (mul_zero_right _)
        ... = n * (m * 0) : {symm (mul_zero_right _)})
    (take l IH,
      calc
        (n * m) * succ l = (n * m) * l + n * m : mul_succ_right _ _
          ... = n * (m * l) + n * m : {IH}
          ... = n * (m * l + m) : symm (mul_add_distr_right _ _ _)
          ... = n * (m * succ l) : {symm (mul_succ_right _ _)})

theorem mul_comm_left (n m k : ℕ) : n * (m * k) = m * (n * k)
:= left_comm mul_comm mul_assoc n m k

theorem mul_comm_right (n m k : ℕ) : n * m * k = n * k * m
:= right_comm mul_comm mul_assoc n m k

theorem mul_one_right (n : ℕ) : n * 1 = n
:= calc
    n * 1 = n * 0 + n : mul_succ_right n 0
      ... = 0 + n : {mul_zero_right n}
      ... = n : add_zero_left n

theorem mul_one_left (n : ℕ) : 1 * n = n
:= calc
    1 * n = n * 1 : mul_comm _ _
      ... = n : mul_one_right n

---------- inversion

theorem mul_eq_zero {n m : ℕ} (H : n * m = 0) : n = 0 ∨ m = 0
:=
  discriminate
    (take Hn : n = 0, or.intro_left _ Hn)
    (take (k : ℕ),
      assume (Hk : n = succ k),
      discriminate
        (take (Hm : m = 0), or.intro_right _ Hm)
        (take (l : ℕ),
          assume (Hl : m = succ l),
          have Heq : succ (k * succ l + l) = n * m, from
            symm (calc
              n * m = n * succ l : { Hl }
                ... = succ k * succ l : { Hk }
                ... = k * succ l + succ l : mul_succ_left _ _
                ... = succ (k * succ l + l) : add_succ_right _ _),
          absurd (trans Heq H) (succ_ne_zero _)))

-- see more under "positivity" below
-------------------------------------------------- le

definition le (n m:ℕ) : Prop := ∃k, n + k = m
infix  `<=`:50 := le
infix  `≤`:50  := le

theorem le_intro {n m k : ℕ} (H : n + k = m) : n ≤ m
:= exists_intro k H

theorem le_elim {n m : ℕ} (H : n ≤ m) : ∃ k, n + k = m
:= H

---------- partial order (totality is part of lt)

theorem le_intro2 (n m : ℕ) : n ≤ n + m
:= le_intro (eq.refl (n + m))

theorem le_refl (n : ℕ) : n ≤ n
:= le_intro (add_zero_right n)

theorem zero_le (n : ℕ) : 0 ≤ n
:= le_intro (add_zero_left n)

theorem le_zero {n : ℕ} (H : n ≤ 0) : n = 0
:=
  obtain (k : ℕ) (Hk : n + k = 0), from le_elim H,
  add_eq_zero_left Hk

theorem not_succ_zero_le (n : ℕ) : ¬ succ n ≤ 0
:= assume H : succ n ≤ 0,
     have H2 : succ n = 0, from le_zero H,
     absurd H2 (succ_ne_zero n)

theorem le_zero_inv {n : ℕ} (H : n ≤ 0) : n = 0
:= obtain (k : ℕ) (Hk : n + k = 0), from le_elim H,
   add_eq_zero_left Hk

theorem le_trans {n m k : ℕ} (H1 : n ≤ m) (H2 : m ≤ k) : n ≤ k
:= obtain (l1 : ℕ) (Hl1 : n + l1 = m), from le_elim H1,
   obtain (l2 : ℕ) (Hl2 : m + l2 = k), from le_elim H2,
   le_intro
     (calc
       n + (l1 + l2) =  n + l1 + l2 : symm (add_assoc n l1 l2)
         ... = m + l2 : { Hl1 }
         ... = k : Hl2)

theorem le_antisym {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 : { symm (add_assoc n k l) }
           ... = m + l : { Hk }
           ... = n : Hl
           ... = n + 0 : symm (add_zero_right n)),
   have L2 : k = 0, from add_eq_zero_left L1,
   calc
     n = n + 0 : symm (add_zero_right n)
      ... = n  + k : { symm L2 }
      ... = m : Hk

---------- interaction with add

theorem add_le_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 n l
               ... = k + m : { Hl })

theorem add_le_right {n m : ℕ} (H : n ≤ m) (k : ℕ) : n + k ≤ m + k
:= (add_comm k m) ▸ (add_comm k n) ▸ (add_le_left H k)

theorem add_le {n m k l : ℕ} (H1 : n ≤ k) (H2 : m ≤ l) : n + m ≤ k + l
:= le_trans (add_le_right H1 m) (add_le_left H2 k)

theorem add_le_left_inv {n m k : 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 : symm (add_assoc k n l)
          ... = k + m : Hl))

theorem add_le_right_inv {n m k : ℕ} (H : n + k ≤ m + k) : n ≤ m
:= add_le_left_inv (add_comm m k ▸ add_comm n k ▸ H)

---------- interaction with succ and pred

theorem succ_le {n m : ℕ} (H : n ≤ m) : succ n ≤ succ m
:= add_one m ▸ add_one n ▸ add_le_right H 1

theorem succ_le_cancel {n m : ℕ} (H : succ n ≤ succ m) :  n ≤ m
:= add_le_right_inv (add_one m⁻¹ ▸ add_one n⁻¹ ▸ H)

theorem self_le_succ (n : ℕ) : n ≤ succ n
:= le_intro (add_one n)

theorem le_imp_le_succ {n m : ℕ} (H : n ≤ m) : n ≤ succ m
:= le_trans H (self_le_succ m)

theorem succ_le_left_or {n m : ℕ} (H : n ≤ m) : succ n ≤ m ∨ n = m
:= obtain (k : ℕ) (Hk : n + k = m), from (le_elim H),
   discriminate
    (assume H3 : k = 0,
      have Heq : n = m,
        from calc
          n = n + 0 : (add_zero_right n)⁻¹
            ... = n + k : {H3⁻¹}
            ... = m : Hk,
      or.intro_right _ Heq)
    (take l:ℕ,
      assume H3 : k = succ l,
      have Hlt : succ n ≤ m, from
        (le_intro
          (calc
            succ n + l = n + succ l : add_move_succ n l
              ... = n + k : {H3⁻¹}
              ... = m : Hk)),
      or.intro_left _ Hlt)

theorem succ_le_left {n m : ℕ} (H1 : n ≤ m) (H2 : n ≠ m) : succ n ≤ m
:= or.resolve_left (succ_le_left_or H1) H2

theorem succ_le_right_inv {n m : ℕ} (H : n ≤ succ m) : n ≤ m ∨ n = succ m
:= or.imp_or (succ_le_left_or H)
     (take H2 : succ n ≤ succ m, show n ≤ m, from succ_le_cancel H2)
     (take H2 : n = succ m, H2)

theorem succ_le_left_inv {n m : ℕ} (H : succ n ≤ m) : n ≤ m ∧ n ≠ m
:= obtain (k : ℕ) (H2 : succ n + k = m), from (le_elim H),
   and.intro
    (have H3 : n + succ k = m,
      from calc
        n + succ k = succ n + k : symm (add_move_succ n k)
          ... = m : H2,
      show n ≤ m, from le_intro H3)
    (assume H3 : n = m,
        have H4 : succ n ≤ n, from subst (symm H3) H,
        have H5 : succ n = n, from le_antisym H4 (self_le_succ n),
        show false, from absurd H5 (succ_ne_self n))

theorem le_pred_self (n : ℕ) : pred n ≤ n
:= case n
    (subst (symm pred_zero) (le_refl 0))
    (take k : ℕ, subst (symm (pred_succ k)) (self_le_succ k))

theorem pred_le {n m : ℕ} (H : n ≤ m) : pred n ≤ pred m
:= discriminate
    (take Hn : n = 0,
      have H2 : pred n = 0,
        from calc
          pred n = pred 0 : {Hn}
             ... = 0 : pred_zero,
      subst (symm H2) (zero_le (pred m)))
    (take k : ℕ,
      assume Hn : n = succ k,
      obtain (l : ℕ) (Hl : n + l = m), from le_elim H,
      have H2 : pred n + l = pred m,
        from calc
          pred n + l = pred (succ k) + l : {Hn}
            ... = k + l : {pred_succ k}
            ... = pred (succ (k + l)) : symm (pred_succ (k + l))
            ... = pred (succ k + l) : {symm (add_succ_left k l)}
            ... = pred (n + l) : {symm Hn}
            ... = pred m : {Hl},
      le_intro H2)

theorem pred_le_left_inv {n m : ℕ} (H : pred n ≤ m) : n ≤ m ∨ n = succ m
:= discriminate
    (take Hn : n = 0,
      or.intro_left _ (subst (symm Hn) (zero_le m)))
    (take k : ℕ,
      assume Hn : n = succ k,
      have H2 : pred n = k,
        from calc
          pred n = pred (succ k) : {Hn}
             ... = k : pred_succ k,
      have H3 : k ≤ m, from subst H2 H,
      have H4 : succ k ≤ m ∨ k = m, from succ_le_left_or H3,
      show n ≤ m ∨ n = succ m, from
        or.imp_or H4
          (take H5 : succ k ≤ m, show n ≤ m, from subst (symm Hn) H5)
          (take H5 : k = m, show n = succ m, from subst H5 Hn))

-- ### interaction with successor and predecessor

theorem le_imp_succ_le_or_eq {n m : ℕ} (H : n ≤ m) : succ n ≤ m ∨ n = m
:=
  obtain (k : ℕ) (Hk : n + k = m), from (le_elim H),
  discriminate
    (assume H3 : k = 0,
      have Heq : n = m,
        from calc
          n = n + 0 : symm (add_zero_right n)
            ... = n + k : {symm H3}
            ... = m : Hk,
      or.intro_right _ Heq)
    (take l : nat,
      assume H3 : k = succ l,
      have Hlt : succ n ≤ m, from
        (le_intro
          (calc
            succ n + l = n + succ l : add_move_succ n l
              ... = n + k : {symm H3}
              ... = m : Hk)),
      or.intro_left _ Hlt)

theorem le_ne_imp_succ_le {n m : ℕ} (H1 : n ≤ m) (H2 : n ≠ m) : succ n ≤ m
:= or.resolve_left (le_imp_succ_le_or_eq H1) H2

theorem le_succ_imp_le_or_eq {n m : ℕ} (H : n ≤ succ m) : n ≤ m ∨ n = succ m
:= or.imp_or_left (le_imp_succ_le_or_eq H)
    (take H2 : succ n ≤ succ m, show n ≤ m, from succ_le_cancel H2)

theorem succ_le_imp_le_and_ne {n m : ℕ} (H : succ n ≤ m) : n ≤ m ∧ n ≠ m
:=
  and.intro
    (le_trans (self_le_succ n) H)
    (assume H2 : n = m,
        have H3 : succ n ≤ n, from subst (symm H2) H,
        have H4 : succ n = n, from le_antisym H3 (self_le_succ n),
        show false, from absurd H4 (succ_ne_self n))

theorem pred_le_self (n : ℕ) : pred n ≤ n
:=
  case n
    (subst (symm pred_zero) (le_refl 0))
    (take k : nat, subst (symm (pred_succ k)) (self_le_succ k))

theorem pred_le_imp_le_or_eq {n m : ℕ} (H : pred n ≤ m) : n ≤ m ∨ n = succ m
:=
  discriminate
    (take Hn : n = 0,
      or.intro_left _ (subst (symm Hn) (zero_le m)))
    (take k : nat,
      assume Hn : n = succ k,
      have H2 : pred n = k,
        from calc
          pred n = pred (succ k) : {Hn}
             ... = k : pred_succ k,
      have H3 : k ≤ m, from subst H2 H,
      have H4 : succ k ≤ m ∨ k = m, from le_imp_succ_le_or_eq H3,
      show n ≤ m ∨ n = succ m, from
        or.imp_or H4
          (take H5 : succ k ≤ m, show n ≤ m, from subst (symm Hn) H5)
          (take H5 : k = m, show n = succ m, from subst H5 Hn))

---------- interaction with mul

theorem mul_le_left {n m : ℕ} (H : n ≤ m) (k : ℕ) : k * n ≤ k * m
:=
  obtain (l : ℕ) (Hl : n + l = m), from (le_elim H),
  induction_on k
    (have H2 : 0 * n = 0 * m,
      from calc
        0 * n = 0 : mul_zero_left n
          ... = 0 * m : symm (mul_zero_left m),
      show 0 * n ≤ 0 * m, from subst H2 (le_refl (0 * n)))
    (take (l : ℕ),
      assume IH : l * n ≤ l * m,
      have H2 : l * n + n ≤ l * m + m, from add_le IH H,
      have H3 : succ l * n ≤ l * m + m, from subst (symm (mul_succ_left l n)) H2,
      show succ l * n ≤ succ l * m, from subst (symm (mul_succ_left l m)) H3)

theorem mul_le_right {n m : ℕ} (H : n ≤ m) (k : ℕ) : n * k ≤ m * k
:= mul_comm k m ▸ mul_comm k n ▸ (mul_le_left H k)

theorem mul_le {n m k l : ℕ} (H1 : n ≤ k) (H2 : m ≤ l) : n * m ≤ k * l
:= le_trans (mul_le_right H1 m) (mul_le_left H2 k)

-- mul_le_[left|right]_inv below

-------------------------------------------------- lt

definition lt (n m : ℕ) := succ n ≤ m
infix `<`:50 := lt

theorem lt_intro {n m k : ℕ} (H : succ n + k = m) : n < m
:= le_intro H

theorem lt_elim {n m : ℕ} (H : n < m) : ∃ k, succ n + k = m
:= le_elim H

theorem lt_intro2 (n m : ℕ) : n < n + succ m
:= lt_intro (add_move_succ n m)

-------------------------------------------------- ge, gt

definition ge (n m : ℕ) := m ≤ n
infix `>=`:50 := ge
infix `≥`:50  := ge

definition gt (n m : ℕ) := m < n
infix `>`:50  := gt

---------- basic facts

theorem lt_ne {n m : ℕ} (H : n < m) : n ≠ m
:= and.elim_right (succ_le_left_inv H)

theorem lt_irrefl (n : ℕ) : ¬ n < n
:= assume H : n < n, absurd (eq.refl n) (lt_ne H)

theorem lt_zero (n : ℕ) : 0 < succ n
:= succ_le (zero_le n)

theorem lt_zero_inv (n : ℕ) : ¬ n < 0
:= assume H : n < 0,
      have H2 : succ n = 0, from le_zero_inv H,
      absurd H2 (succ_ne_zero n)

theorem lt_positive {n m : ℕ} (H : n < m) : ∃k, m = succ k
:= discriminate
    (take (Hm : m = 0), absurd (subst Hm H) (lt_zero_inv n))
    (take (l : ℕ) (Hm : m = succ l), exists_intro l Hm)

---------- interaction with le

theorem lt_imp_le_succ {n m : ℕ} (H : n < m) : succ n ≤ m
:= H

theorem le_succ_imp_lt {n m : ℕ} (H : succ n ≤ m) : n < m
:= H

theorem self_lt_succ (n : ℕ) : n < succ n
:= le_refl (succ n)

theorem lt_imp_le {n m : ℕ} (H : n < m) : n ≤ m
:= and.elim_left (succ_le_imp_le_and_ne H)

theorem le_imp_lt_or_eq {n m : ℕ} (H : n ≤ m) : n < m ∨ n = m
:= le_imp_succ_le_or_eq H

theorem le_ne_imp_lt {n m : ℕ} (H1 : n ≤ m) (H2 : n ≠ m) : n < m
:= le_ne_imp_succ_le H1 H2

theorem le_imp_lt_succ {n m : ℕ} (H : n ≤ m) : n < succ m
:= succ_le H

theorem lt_succ_imp_le {n m : ℕ} (H : n < succ m) : n ≤ m
:= succ_le_cancel H

---------- trans, antisym

theorem lt_le_trans {n m k : ℕ} (H1 : n < m) (H2 : m ≤ k) : n < k
:= le_trans H1 H2

theorem le_lt_trans {n m k : ℕ} (H1 : n ≤ m) (H2 : m < k) : n < k
:= le_trans (succ_le H1) H2

theorem lt_trans {n m k : ℕ} (H1 : n < m) (H2 : m < k) : n < k
:= lt_le_trans H1 (lt_imp_le H2)

theorem le_imp_not_gt {n m : ℕ} (H : n ≤ m) : ¬ n > m
:= assume H2 : m < n, absurd (le_lt_trans H H2) (lt_irrefl n)

theorem lt_imp_not_ge {n m : ℕ} (H : n < m) : ¬ n ≥ m
:= assume H2 : m ≤ n, absurd (lt_le_trans H H2) (lt_irrefl n)

theorem lt_antisym {n m : ℕ} (H : n < m) : ¬ m < n
:= le_imp_not_gt (lt_imp_le H)

---------- interaction with add

theorem add_lt_left {n m : ℕ} (H : n < m) (k : ℕ) : k + n < k + m
:= add_succ_right k n ▸ add_le_left H k

theorem add_lt_right {n m : ℕ} (H : n < m) (k : ℕ) : n + k < m + k
:= add_comm k m ▸ add_comm k n ▸ add_lt_left H k

theorem add_le_lt {n m k l : ℕ} (H1 : n ≤ k) (H2 : m < l) : n + m < k + l
:= le_lt_trans (add_le_right H1 m) (add_lt_left H2 k)

theorem add_lt_le {n m k l : ℕ} (H1 : n < k) (H2 : m ≤ l) : n + m < k + l
:= lt_le_trans (add_lt_right H1 m) (add_le_left H2 k)

theorem add_lt {n m k l : ℕ} (H1 : n < k) (H2 : m < l) : n + m < k + l
:= add_lt_le H1 (lt_imp_le H2)

theorem add_lt_left_inv {n m k : ℕ} (H : k + n < k + m) : n < m
:= add_le_left_inv (add_succ_right k n⁻¹ ▸ H)

theorem add_lt_right_inv {n m k : ℕ} (H : n + k < m + k) : n < m
:= add_lt_left_inv (add_comm m k ▸ add_comm n k ▸ H)

---------- interaction with succ (see also the interaction with le)

theorem succ_lt {n m : ℕ} (H : n < m) : succ n < succ m
:= add_one m ▸ add_one n ▸ add_lt_right H 1

theorem succ_lt_inv {n m : ℕ} (H : succ n < succ m) :  n < m
:= add_lt_right_inv (add_one m⁻¹ ▸ add_one n⁻¹ ▸ H)

theorem lt_self_succ (n : ℕ) : n < succ n
:= le_refl (succ n)

theorem succ_lt_right {n m : ℕ} (H : n < m) : n < succ m
:= lt_trans H (lt_self_succ m)

---------- totality of lt and le

theorem le_or_lt (n m : ℕ) : n ≤ m ∨ m < n
:= induction_on n
    (or.intro_left _ (zero_le m))
    (take (k : ℕ),
      assume IH : k ≤ m ∨ m < k,
      or.elim IH
        (assume H : k ≤ m,
          obtain (l : ℕ) (Hl : k + l = m), from le_elim H,
          discriminate
            (assume H2 : l = 0,
              have H3 : m = k,
                from calc
                  m = k + l : symm Hl
                    ... = k + 0 : {H2}
                    ... = k : add_zero_right k,
              have H4 : m < succ k, from subst  H3 (lt_self_succ m),
              or.intro_right _ H4)
            (take l2 : ℕ,
              assume H2 : l = succ l2,
              have H3 : succ k + l2 = m,
                from calc
                  succ k + l2 = k + succ l2 : add_move_succ k l2
                    ... = k + l : {symm H2}
                    ... = m : Hl,
              or.intro_left _ (le_intro H3)))
        (assume H : m < k, or.intro_right _ (succ_lt_right H)))

theorem trichotomy_alt (n m : ℕ) : (n < m ∨ n = m) ∨ m < n
:= or.imp_or (le_or_lt n m) (assume H : n ≤ m, le_imp_lt_or_eq H) (assume H : m < n, H)

theorem trichotomy (n m : ℕ) : n < m ∨ n = m ∨ m < n
:= iff.elim_left or.assoc (trichotomy_alt n m)

theorem le_total (n m : ℕ) : n ≤ m ∨ m ≤ n
:= or.imp_or (le_or_lt n m) (assume H : n ≤ m, H) (assume H : m < n, lt_imp_le H)

-- interaction with mul under "positivity"

theorem strong_induction_on {P : ℕ → Prop} (n : ℕ) (IH : ∀n, (∀m, m < n → P m) → P n) : P n
:= have stronger : ∀k, k ≤ n → P k, from
    induction_on n
      (take (k : ℕ),
        assume H : k ≤ 0,
        have H2 : k = 0, from le_zero_inv H,
        have H3 : ∀m, m < k → P m, from
          (take m : ℕ,
            assume H4 : m < k,
            have H5 : m < 0, from subst H2 H4,
            absurd H5 (lt_zero_inv m)),
        show P k, from IH k H3)
      (take l : ℕ,
        assume IHl : ∀k, k ≤ l → P k,
        take k : ℕ,
        assume H : k ≤ succ l,
        or.elim (succ_le_right_inv H)
          (assume H2 : k ≤ l, show P k, from IHl k H2)
          (assume H2 : k = succ l,
            have H3 : ∀m, m < k → P m, from
              (take m : ℕ,
                assume H4 : m < k,
                have H5 : m ≤ l, from lt_succ_imp_le (subst H2 H4),
                show P m, from IHl m H5),
            show P k, from IH k H3)),
  stronger n (le_refl n)

theorem case_strong_induction_on {P : ℕ → Prop} (a : ℕ) (H0 : P 0) (Hind : ∀(n : ℕ), (∀m, m ≤ n → P m) → P (succ n)) : P a
:= strong_induction_on a
     (take n, case n
       (assume H : (∀m, m < 0 → P m), H0)
       (take n, assume H : (∀m, m < succ n → P m),
         Hind n (take m, assume H1 : m ≤ n, H m (le_imp_lt_succ H1))))

theorem add_eq_self {n m : ℕ} (H : n + m = n) : m = 0
:= discriminate
    (take Hm : m = 0, Hm)
    (take k : ℕ,
      assume Hm : m = succ k,
      have H2 : succ n + k = n,
        from calc
          succ n + k = n + succ k : add_move_succ n k
            ... = n + m : {symm Hm}
            ... = n : H,
      have H3 : n < n, from lt_intro H2,
      have H4 : n ≠ n, from lt_ne H3,
      absurd (eq.refl n) H4)

-------------------------------------------------- positivity

--  we use " _ > 0" as canonical way of denoting that a number is positive

---------- basic

theorem zero_or_positive (n : ℕ) : n = 0 ∨ n > 0
:= or.imp_or (or.swap (le_imp_lt_or_eq (zero_le n))) (take H : 0 = n, symm H) (take H : n > 0, H)

theorem succ_positive {n m : ℕ} (H : n = succ m) : n > 0
:= subst (symm H) (lt_zero m)

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

theorem pos_imp_eq_succ {n : ℕ} (H : n > 0) : ∃l, n = succ l
:= discriminate
    (take H2, absurd (subst H2 H) (lt_irrefl 0))
    (take l Hl, exists_intro l Hl)

theorem add_positive_right (n : ℕ) {k : ℕ} (H : k > 0) : n + k > n
:= obtain (l : ℕ) (Hl : k = succ l), from pos_imp_eq_succ H,
   subst (symm Hl) (lt_intro2 n l)

theorem add_positive_left (n : ℕ) {k : ℕ} (H : k > 0) : k + n > n
:= subst (add_comm n k) (add_positive_right n H)


-- Positivity
-- ---------
--
-- Writing "t > 0" is the preferred way to assert that a natural number is positive.

-- ### basic

-- See also succ_pos.

theorem succ_pos (n : ℕ) : 0 < succ n
:= succ_le (zero_le n)

theorem case_zero_pos {P : ℕ → Prop} (y : ℕ) (H0 : P 0) (H1 : ∀y, y > 0 → P y) : P y
:= case y H0 (take y', H1 _ (succ_pos _))

theorem zero_or_pos (n : ℕ) : n = 0 ∨ n > 0
:= or.imp_or_left (or.swap (le_imp_lt_or_eq (zero_le n))) (take H : 0 = n, symm H)

theorem succ_imp_pos {n m : ℕ} (H : n = succ m) : n > 0
:= subst (symm H) (succ_pos m)

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

theorem add_pos_right (n : ℕ) {k : ℕ} (H : k > 0) : n + k > n
:= subst (add_zero_right n) (add_lt_left H n)

theorem add_pos_left (n : ℕ) {k : ℕ} (H : k > 0) : k + n > n
:= subst (add_comm n k) (add_pos_right n H)

---------- mul

theorem mul_positive {n m : ℕ} (Hn : n > 0) (Hm : m > 0) : n * m > 0
:= obtain (k : ℕ) (Hk : n = succ k), from pos_imp_eq_succ Hn,
   obtain (l : ℕ) (Hl : m = succ l), from pos_imp_eq_succ Hm,
   succ_positive (calc
     n * m = succ k * m : {Hk}
       ... = succ k * succ l : {Hl}
       ... = succ k * l + succ k : mul_succ_right (succ k) l
       ... = succ (succ k * l + k) : add_succ_right _ _)

theorem mul_positive_inv_left {n m : ℕ} (H : n * m > 0) : n > 0
:= discriminate
    (assume H2 : n = 0,
      have H3 : n * m = 0,
        from calc
          n * m = 0 * m : {H2}
            ... = 0 : mul_zero_left m,
      have H4 : 0 > 0, from subst H3 H,
      absurd H4 (lt_irrefl 0))
    (take l : ℕ,
      assume Hl : n = succ l,
      subst (symm Hl) (lt_zero l))

theorem mul_positive_inv_right {n m : ℕ} (H : n * m > 0) : m > 0
:= mul_positive_inv_left (subst (mul_comm n m) H)

theorem mul_left_inj {n m k : ℕ} (Hn : n > 0) (H : n * m = n * k) : m = k
:=
  have general : ∀m, n * m = n * k → m = k, from
    induction_on k
      (take m:ℕ,
        assume H : n * m = n * 0,
        have H2 : n * m = 0,
          from calc
            n * m = n * 0 : H
              ... = 0 : mul_zero_right n,
        have H3 : n = 0 ∨ m = 0, from mul_eq_zero H2,
        or.resolve_right H3 (ne.symm (lt_ne Hn)))
      (take (l : ℕ),
        assume (IH : ∀ m, n * m = n * l → m = l),
        take (m : ℕ),
        assume (H : n * m = n * succ l),
        have H2 :  n * succ l > 0, from mul_positive Hn (lt_zero l),
        have H3 : m > 0, from mul_positive_inv_right (subst (symm H) H2),
        obtain (l2:ℕ) (Hm : m = succ l2), from pos_imp_eq_succ H3,
        have H4 : n * l2 + n = n * l + n,
          from calc
            n * l2 + n = n * succ l2 : symm (mul_succ_right n l2)
              ... = n * m : {symm Hm}
              ... = n * succ l : H
              ... = n * l + n : mul_succ_right n l,
        have H5 : n * l2 = n * l, from add_cancel_right H4,
        calc
          m = succ l2 : Hm
        ... = succ l : {IH l2 H5}),
  general m H

theorem mul_right_inj {n m k : ℕ} (Hm : m > 0) (H : n * m = k * m) : n = k
:= mul_left_inj Hm (subst (mul_comm k m) (subst (mul_comm n m) H))

-- mul_eq_one below

---------- interaction of mul with le and lt


theorem mul_lt_left {n m k : ℕ} (Hk : k > 0) (H : n < m) : k * n < k * m
:=
  have H2 : k * n < k * n + k, from add_positive_right (k * n) Hk,
  have H3 : k * n + k ≤ k * m, from subst (mul_succ_right k n) (mul_le_left H k),
  lt_le_trans H2 H3

theorem mul_lt_right {n m k : ℕ} (Hk : k > 0) (H : n < m)  : n * k < m * k
:= subst (mul_comm k m) (subst (mul_comm k n) (mul_lt_left Hk H))

theorem mul_le_lt {n m k l : ℕ} (Hk : k > 0) (H1 : n ≤ k) (H2 : m < l) : n * m < k * l
:= le_lt_trans (mul_le_right H1 m) (mul_lt_left Hk H2)

theorem mul_lt_le {n m k l : ℕ} (Hl : l > 0) (H1 : n < k) (H2 : m ≤ l) : n * m < k * l
:= le_lt_trans (mul_le_left H2 n) (mul_lt_right Hl H1)

theorem mul_lt {n m k l : ℕ} (H1 : n < k) (H2 : m < l) : n * m < k * l
:=
  have H3 : n * m ≤ k * m, from mul_le_right (lt_imp_le H1) m,
  have H4 : k * m < k * l, from mul_lt_left (le_lt_trans (zero_le n) H1) H2,
  le_lt_trans H3 H4

theorem mul_lt_left_inv {n m k : ℕ} (H : k * n < k * m) : n < m
:=
  have general : ∀ m, k * n < k * m → n < m, from
    induction_on n
      (take m : ℕ,
        assume H2 : k * 0 < k * m,
        have H3 : 0 < k * m, from mul_zero_right k ▸ H2,
        show 0 < m, from mul_positive_inv_right H3)
      (take l : ℕ,
        assume IH : ∀ m, k * l < k * m → l < m,
        take m : ℕ,
        assume H2 : k * succ l < k * m,
        have H3 : 0 < k * m, from le_lt_trans (zero_le _) H2,
        have H4 : 0 < m, from mul_positive_inv_right H3,
        obtain (l2 : ℕ) (Hl2 : m = succ l2), from pos_imp_eq_succ H4,
        have H5 : k * l + k < k * m, from mul_succ_right k l ▸ H2,
        have H6 : k * l + k < k * succ l2, from Hl2 ▸ H5,
        have H7 : k * l + k < k * l2 + k, from mul_succ_right k l2 ▸ H6,
        have H8 : k * l < k * l2, from add_lt_right_inv H7,
        have H9 : l < l2, from IH l2 H8,
        have H10 : succ l < succ l2, from succ_lt H9,
        show succ l < m, from Hl2⁻¹ ▸ H10),
  general m H

theorem mul_lt_right_inv {n m k : ℕ} (H : n * k < m * k) : n < m
:= mul_lt_left_inv (mul_comm m k ▸ mul_comm n k ▸ H)

theorem mul_le_left_inv {n m k : ℕ} (H : succ k * n ≤ succ k * m) : n ≤ m
:=
  have H2 : succ k * n < succ k * m + succ k, from le_lt_trans H (lt_intro2 _ _),
  have H3 : succ k * n < succ k * succ m, from subst (symm (mul_succ_right (succ k) m)) H2,
  have H4 : n < succ m, from mul_lt_left_inv H3,
  show n ≤ m, from lt_succ_imp_le H4

theorem mul_le_right_inv {n m k : ℕ} (H : n * succ m ≤ k * succ m) : n ≤ k
:= mul_le_left_inv (subst (mul_comm k (succ m)) (subst (mul_comm n (succ m)) H))

theorem mul_eq_one_left {n m : ℕ} (H : n * m = 1) : n = 1
:=
  have H2 : n * m > 0, from subst (symm H) (lt_zero 0),
  have H3 : n > 0, from mul_positive_inv_left H2,
  have H4 : m > 0, from mul_positive_inv_right H2,
  or.elim (le_or_lt n 1)
    (assume H5 : n ≤ 1,
      show n = 1, from le_antisym H5 H3)
    (assume H5 : n > 1,
      have H6 : n * m ≥ 2 * 1, from mul_le H5 H4,
      have H7 : 1 ≥ 2, from subst (mul_one_right 2) (subst H H6),
      absurd (self_lt_succ 1) (le_imp_not_gt H7))

theorem mul_eq_one_right {n m : ℕ} (H : n * m = 1) : m = 1
:= mul_eq_one_left (subst (mul_comm n m) H)

theorem mul_eq_one {n m : ℕ} (H : n * m = 1) : n = 1 ∧ m = 1
:= and.intro (mul_eq_one_left H) (mul_eq_one_right H)

-------------------------------------------------- sub

definition sub (n m : ℕ) : ℕ := nat.rec n (fun m x, pred x) m
infixl `-`:65 := sub
theorem sub_zero_right (n : ℕ) : n - 0 = n
theorem sub_succ_right (n m : ℕ) : n - succ m = pred (n - m)

theorem sub_zero_left (n : ℕ) : 0 - n = 0
:= induction_on n (sub_zero_right 0)
    (take k : ℕ,
      assume IH : 0 - k = 0,
      calc
        0 - succ k = pred (0 - k) : sub_succ_right 0 k
          ... = pred 0 : {IH}
          ... = 0 : pred_zero)

theorem sub_succ_succ (n m : ℕ) : succ n - succ m = n - m
:= induction_on m
    (calc
      succ n - 1 = pred (succ n - 0) : sub_succ_right (succ n) 0
        ... = pred (succ n) : {sub_zero_right (succ n)}
        ... = n : pred_succ n
        ... = n - 0 : symm (sub_zero_right n))
    (take k : ℕ,
      assume IH : succ n - succ k = n - k,
      calc
        succ n - succ (succ k) = pred (succ n - succ k) : sub_succ_right (succ n) (succ k)
          ... = pred (n - k) : {IH}
          ... = n - succ k : symm (sub_succ_right n k))

theorem sub_one (n : ℕ) : n - 1 = pred n
:= calc
    n - 1 = pred (n - 0) : sub_succ_right n 0
      ... = pred n : {sub_zero_right n}

theorem sub_self (n : ℕ) : n - n = 0
:= induction_on n (sub_zero_right 0) (take k IH, trans (sub_succ_succ k k) IH)

theorem sub_add_add_right (n m k : ℕ) : (n + k) - (m + k) = n - m
:= induction_on k
    (calc
      (n + 0) - (m + 0) = n - (m + 0) : {add_zero_right _}
        ... = n - m : {add_zero_right _})
    (take l : ℕ,
      assume IH : (n + l) - (m + l) = n - m,
      calc
        (n + succ l) - (m + succ l) = succ (n + l) - (m + succ l) : {add_succ_right _ _}
          ... = succ (n + l) - succ (m + l) : {add_succ_right _ _}
          ... = (n + l) - (m + l) : sub_succ_succ _ _
          ... =  n - m : IH)

theorem sub_add_add_left (n m k : ℕ) : (k + n) - (k + m) = n - m
:= subst (add_comm m k) (subst (add_comm n k) (sub_add_add_right n m k))

theorem sub_add_left (n m : ℕ) : n + m - m = n
:= induction_on m
    (subst (symm (add_zero_right n)) (sub_zero_right n))
    (take k : ℕ,
      assume IH : n + k - k = n,
      calc
        n + succ k - succ k = succ (n + k) - succ k : {add_succ_right n k}
          ... = n + k - k : sub_succ_succ _ _
          ... = n : IH)

theorem sub_sub (n m k : ℕ) : n - m - k = n - (m + k)
:= induction_on k
    (calc
      n - m - 0 = n - m : sub_zero_right _
        ... =  n - (m + 0) : {symm (add_zero_right m)})
    (take l : ℕ,
      assume IH : n - m - l = n - (m + l),
      calc
        n - m - succ l = pred (n - m - l) : sub_succ_right (n - m) l
          ... = pred (n - (m + l)) : {IH}
          ... = n - succ (m + l) : symm (sub_succ_right n (m + l))
          ... = n - (m + succ l) : {symm (add_succ_right m l)})

theorem succ_sub_sub (n m k : ℕ) : succ n - m - succ k = n - m - k
:= calc
    succ n - m - succ k = succ n - (m + succ k) : sub_sub _ _ _
      ... = succ n - succ (m + k) : {add_succ_right m k}
      ... = n - (m + k) : sub_succ_succ _ _
      ... = n - m - k : symm (sub_sub n m k)

theorem sub_add_right_eq_zero (n m : ℕ) : n - (n + m) = 0
:= calc
    n - (n + m) = n - n - m : symm (sub_sub n n m)
      ... = 0 - m : {sub_self n}
      ... = 0 : sub_zero_left m

theorem sub_comm (m n k : ℕ) : m - n - k = m - k - n
:= calc
    m - n - k = m - (n + k) : sub_sub m n k
      ... = m - (k + n) : {add_comm n k}
      ... = m - k - n : symm (sub_sub m k n)

theorem succ_sub_one (n : ℕ) : succ n - 1 = n
:= sub_succ_succ n 0 ⬝ sub_zero_right n

---------- mul

theorem mul_pred_left (n m : ℕ) : pred n * m = n * m - m
:= induction_on n
    (calc
      pred 0 * m = 0 * m : {pred_zero}
        ... = 0 : mul_zero_left _
        ... = 0 - m : symm (sub_zero_left m)
        ... = 0 * m - m : {symm (mul_zero_left m)})
    (take k : ℕ,
      assume IH : pred k * m = k * m - m,
      calc
        pred (succ k) * m = k * m : {pred_succ k}
          ... = k * m + m - m : symm (sub_add_left _ _)
          ... = succ k * m - m : {symm (mul_succ_left k m)})

theorem mul_pred_right (n m : ℕ) : n * pred m = n * m - n
:= calc n * pred m = pred m * n : mul_comm _ _
    ... = m * n - n : mul_pred_left m n
    ... = n * m - n : {mul_comm m n}

theorem mul_sub_distr_left (n m k : ℕ) : (n - m) * k = n * k - m * k
:= induction_on m
    (calc
      (n - 0) * k = n * k : {sub_zero_right n}
        ... = n * k - 0 : symm (sub_zero_right _)
        ... = n * k - 0 * k : {symm (mul_zero_left _)})
    (take l : ℕ,
      assume IH : (n - l) * k = n * k - l * k,
      calc
        (n - succ l) * k = pred (n - l) * k : {sub_succ_right n l}
          ... = (n - l) * k - k : mul_pred_left _ _
          ... = n * k - l * k - k : {IH}
          ... = n * k - (l * k + k) : sub_sub _ _ _
          ... = n * k - (succ l * k) : {symm (mul_succ_left l k)})

theorem mul_sub_distr_right (n m k : ℕ) : n * (m - k) = n * m - n * k
:= calc
    n * (m - k) = (m - k) * n : mul_comm _ _
      ... = m * n - k * n : mul_sub_distr_left _ _ _
      ... = n * m - k * n : {mul_comm _ _}
      ... = n * m - n * k : {mul_comm _ _}

-------------------------------------------------- max, min, iteration, maybe: sub, div

theorem succ_sub {m n : ℕ} : m ≥ n → succ m - n  = succ (m - n)
:= sub_induction n m
    (take k,
      assume H : 0 ≤ k,
      calc
        succ k - 0 = succ k : sub_zero_right (succ k)
          ... = succ (k - 0) : {symm (sub_zero_right k)})
    (take k,
      assume H : succ k ≤ 0,
      absurd H (not_succ_zero_le k))
    (take k l,
      assume IH : k ≤ l → succ l - k = succ (l - k),
      take H : succ k ≤ succ l,
      calc
        succ (succ l) - succ k = succ l - k : sub_succ_succ (succ l) k
          ... = succ (l - k) : IH (succ_le_cancel H)
          ... = succ (succ l - succ k) : {symm (sub_succ_succ l k)})

theorem le_imp_sub_eq_zero {n m : ℕ} (H : n ≤ m) : n - m = 0
:= obtain (k : ℕ) (Hk : n + k = m), from le_elim H, subst Hk (sub_add_right_eq_zero n k)

theorem add_sub_le {n m : ℕ} : n ≤ m → n + (m - n) = m
:= sub_induction n m
    (take k,
      assume H : 0 ≤ k,
      calc
        0 + (k - 0) = k - 0 : add_zero_left (k - 0)
          ... = k : sub_zero_right k)
    (take k, assume H : succ k ≤ 0, absurd H (not_succ_zero_le k))
    (take k l,
      assume IH : k ≤ l → k + (l - k) = l,
      take H : succ k ≤ succ l,
      calc
        succ k + (succ l - succ k) = succ k + (l - k) : {sub_succ_succ l k}
          ... = succ (k + (l - k)) : add_succ_left k (l - k)
          ... = succ l : {IH (succ_le_cancel H)})

theorem add_sub_ge_left {n m : ℕ} : n ≥ m → n - m + m = n
:= subst (add_comm m (n - m)) add_sub_le

theorem add_sub_ge {n m : ℕ} (H : n ≥ m) : n + (m - n) = n
:= calc
    n + (m - n) = n + 0 : {le_imp_sub_eq_zero H}
      ... = n : add_zero_right n

theorem add_sub_le_left {n m : ℕ} : n ≤ m → n - m + m = m
:= subst (add_comm m (n - m)) add_sub_ge

theorem le_add_sub_left (n m : ℕ) : n ≤ n + (m - n)
:= or.elim (le_total n m)
    (assume H : n ≤ m, subst (symm (add_sub_le H)) H)
    (assume H : m ≤ n, subst (symm (add_sub_ge H)) (le_refl n))

theorem le_add_sub_right (n m : ℕ) : m ≤ n + (m - n)
:= or.elim (le_total n m)
    (assume H : n ≤ m, subst (symm (add_sub_le H)) (le_refl m))
    (assume H : m ≤ n, subst (symm (add_sub_ge H)) H)

theorem sub_split {P : ℕ → Prop} {n m : ℕ} (H1 : n ≤ m → P 0) (H2 : ∀k, m + k = n -> P k)
    : P (n - m)
:= or.elim (le_total n m)
    (assume H3 : n ≤ m, subst (symm (le_imp_sub_eq_zero H3)) (H1 H3))
    (assume H3 : m ≤ n, H2 (n - m) (add_sub_le H3))

theorem sub_le_self (n m : ℕ) : n - m ≤ n
:=
  sub_split
    (assume H : n ≤ m, zero_le n)
    (take k : ℕ, assume H : m + k = n, le_intro (subst (add_comm m k) H))

theorem le_elim_sub (n m : ℕ) (H : n ≤ m) : ∃k, m - k = n
:=
  obtain (k : ℕ) (Hk : n + k = m), from le_elim H,
  exists_intro k
    (calc
      m - k = n + k - k : {symm Hk}
        ... = n : sub_add_left n k)

theorem add_sub_assoc {m k : ℕ} (H : k ≤ m) (n : ℕ) : n + m - k = n + (m - k)
:= have l1 : k ≤ m → n + m - k = n + (m - k), from
    sub_induction k m
      (take m : ℕ,
        assume H : 0 ≤ m,
        calc
          n + m - 0 = n + m : sub_zero_right (n + m)
            ... = n + (m - 0) : {symm (sub_zero_right m)})
      (take k : ℕ, assume H : succ k ≤ 0, absurd H (not_succ_zero_le k))
      (take k m,
        assume IH : k ≤ m → n + m - k = n + (m - k),
        take H : succ k ≤ succ m,
        calc
          n + succ m - succ k = succ (n + m) - succ k : {add_succ_right n m}
            ... = n + m - k : sub_succ_succ (n + m) k
            ... = n + (m - k) : IH (succ_le_cancel H)
            ... = n + (succ m - succ k) : {symm (sub_succ_succ m k)}),
  l1 H

theorem sub_eq_zero_imp_le {n m : ℕ} : n - m = 0 → n ≤ m
:= sub_split
    (assume H1 : n ≤ m, assume H2 : 0 = 0, H1)
    (take k : ℕ,
      assume H1 : m + k = n,
      assume H2 : k = 0,
      have H3 : n = m, from subst (add_zero_right m) (subst H2 (symm H1)),
      subst H3 (le_refl n))

theorem sub_sub_split {P : ℕ → ℕ → Prop} {n m : ℕ} (H1 : ∀k, n = m + k -> P k 0)
    (H2 : ∀k, m = n + k → P 0 k) : P (n - m) (m - n)
:= or.elim (le_total n m)
    (assume H3 : n ≤ m,
      le_imp_sub_eq_zero H3⁻¹ ▸  (H2 (m - n) (add_sub_le H3⁻¹)))
    (assume H3 : m ≤ n,
      le_imp_sub_eq_zero H3⁻¹ ▸ (H1 (n - m) (add_sub_le H3⁻¹)))

theorem sub_intro {n m k : ℕ} (H : n + m = k) : k - n = m
:= have H2 : k - n + n = m + n, from
    calc
      k - n + n = k : add_sub_ge_left (le_intro H)
        ... = n + m : symm H
        ... = m + n : add_comm n m,
   add_cancel_right H2

theorem sub_lt {x y : ℕ} (xpos : x > 0) (ypos : y > 0) : x - y < x
:= obtain (x' : ℕ) (xeq : x = succ x'), from pos_imp_eq_succ xpos,
   obtain (y' : ℕ) (yeq : y = succ y'), from pos_imp_eq_succ ypos,
   have xsuby_eq : x - y = x' - y', from
     calc
       x - y = succ x' - y : {xeq}
         ... = succ x' - succ y' : {yeq}
         ... = x' - y' : sub_succ_succ _ _,
   have H1 : x' - y' ≤ x', from sub_le_self _ _,
   have H2 : x' < succ x', from self_lt_succ _,
   show x - y < x, from xeq⁻¹ ▸ xsuby_eq⁻¹ ▸ le_lt_trans H1 H2

-- Max, min, iteration, and absolute difference
-- --------------------------------------------

definition max (n m : ℕ) : ℕ := n + (m - n)
definition min (n m : ℕ) : ℕ := m - (m - n)

theorem max_le {n m : ℕ} (H : n ≤ m) : n + (m - n) = m := add_sub_le H

theorem max_ge {n m : ℕ} (H : n ≥ m) : n + (m - n) = n := add_sub_ge H

theorem left_le_max (n m : ℕ) : n ≤ n + (m - n) := le_add_sub_left n m

theorem right_le_max (n m : ℕ) : m ≤ max n m := le_add_sub_right n m

-- ### absolute difference

-- This section is still incomplete

definition dist (n m : ℕ) := (n - m) + (m - n)

theorem dist_comm (n m : ℕ) : dist n m = dist m n
:= add_comm (n - m) (m - n)

theorem dist_eq_zero {n m : ℕ} (H : dist n m = 0) : n = m
:=
  have H2 : n - m = 0, from add_eq_zero_left H,
  have H3 : n ≤ m, from sub_eq_zero_imp_le H2,
  have H4 : m - n = 0, from add_eq_zero_right H,
  have H5 : m ≤ n, from sub_eq_zero_imp_le H4,
  le_antisym H3 H5

theorem dist_le {n m : ℕ} (H : n ≤ m) : dist n m = m - n
:= calc
    dist n m = (n - m) + (m - n) : eq.refl _
         ... = 0       + (m - n) : {le_imp_sub_eq_zero H}
         ... = m - n             : add_zero_left (m - n)

theorem dist_ge {n m : ℕ} (H : n ≥ m) : dist n m = n - m
:= subst (dist_comm m n) (dist_le H)

theorem dist_zero_right (n : ℕ) : dist n 0 = n
:= trans (dist_ge (zero_le n)) (sub_zero_right n)

theorem dist_zero_left (n : ℕ) : dist 0 n = n
:= trans (dist_le (zero_le n)) (sub_zero_right n)

theorem dist_intro {n m k : ℕ} (H : n + m = k) : dist k n = m
:= calc
    dist k n = k - n : dist_ge (le_intro H)
      ... = m : sub_intro H

theorem dist_add_right (n k m : ℕ) : dist (n + k) (m + k) = dist n m
:=
  calc
    dist (n + k) (m + k) = ((n+k) - (m+k)) + ((m+k)-(n+k)) : eq.refl _
                     ... = (n - m) + ((m + k) - (n + k))   : {sub_add_add_right _ _ _}
                     ... = (n - m) + (m - n)               : {sub_add_add_right _ _ _}

theorem dist_add_left (k n m : ℕ) : dist (k + n) (k + m) = dist n m
:= subst (add_comm m k) (subst (add_comm n k) (dist_add_right n k m))

theorem dist_ge_add_right {n m : ℕ} (H : n ≥ m) : dist n m + m = n
:= calc
    dist n m + m = n - m + m : {dist_ge H}
      ... = n : add_sub_ge_left H

theorem dist_eq_intro {n m k l : ℕ} (H : n + m = k + l) : dist n k = dist l m
:= calc
    dist n k = dist (n + m) (k + m) : symm (dist_add_right n m k)
      ... = dist (k + l) (k + m) : {H}
      ... = dist l m : dist_add_left k l m



end nat
