/-
Copyright (c) 2014 Jeremy Avigad. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Jeremy Avigad, Leonardo de Moura

Various multiplicative and additive structures. Partially modeled on Isabelle's library.
-/

import logic.eq data.unit data.sigma data.prod
import algebra.binary algebra.priority

open binary

variable {A : Type}

/- semigroup -/

attribute inv [light 3]
attribute neg [light 3]

structure semigroup [class] (A : Type) extends has_mul A :=
(mul_assoc : ∀a b c, mul (mul a b) c = mul a (mul b c))

-- We add pattern hints to the following lemma because we want it to be used in both directions
-- at inst_simp strategy.
theorem mul.assoc [simp] [semigroup A] (a b c : A) : (: a * b * c :) = (: a * (b * c) :) :=
!semigroup.mul_assoc

structure comm_semigroup [class] (A : Type) extends semigroup A :=
(mul_comm : ∀a b, mul a b = mul b a)

theorem mul.comm [simp] [comm_semigroup A] (a b : A) : a * b = b * a :=
!comm_semigroup.mul_comm

theorem mul.left_comm [simp] [comm_semigroup A] (a b c : A) : a * (b * c) = b * (a * c) :=
binary.left_comm (@mul.comm A _) (@mul.assoc A _) a b c

theorem mul.right_comm [comm_semigroup A] (a b c : A) : (a * b) * c = (a * c) * b :=
by simp

structure left_cancel_semigroup [class] (A : Type) extends semigroup A :=
(mul_left_cancel : ∀a b c, mul a b = mul a c → b = c)

theorem mul.left_cancel [left_cancel_semigroup A] {a b c : A} : a * b = a * c → b = c :=
!left_cancel_semigroup.mul_left_cancel

abbreviation eq_of_mul_eq_mul_left' := @mul.left_cancel

structure right_cancel_semigroup [class] (A : Type) extends semigroup A :=
(mul_right_cancel : ∀a b c, mul a b = mul c b → a = c)

theorem mul.right_cancel [right_cancel_semigroup A] {a b c : A} : a * b = c * b → a = c :=
!right_cancel_semigroup.mul_right_cancel

abbreviation eq_of_mul_eq_mul_right' := @mul.right_cancel

/- additive semigroup -/

structure add_semigroup [class] (A : Type) extends has_add A :=
(add_assoc : ∀a b c, add (add a b) c = add a (add b c))

theorem add.assoc [simp] [add_semigroup A] (a b c : A) : (: a + b + c :) = (: a + (b + c) :) :=
!add_semigroup.add_assoc

structure add_comm_semigroup [class] (A : Type) extends add_semigroup A :=
(add_comm : ∀a b, add a b = add b a)

theorem add.comm [simp] [add_comm_semigroup A] (a b : A) : a + b = b + a :=
!add_comm_semigroup.add_comm

theorem add.left_comm [simp] [add_comm_semigroup A] (a b c : A) : a + (b + c) = b + (a + c) :=
binary.left_comm (@add.comm A _) (@add.assoc A _) a b c

theorem add.right_comm [add_comm_semigroup A] (a b c : A) : (a + b) + c = (a + c) + b :=
by simp

structure add_left_cancel_semigroup [class] (A : Type) extends add_semigroup A :=
(add_left_cancel : ∀a b c, add a b = add a c → b = c)

theorem add.left_cancel [add_left_cancel_semigroup A] {a b c : A} : a + b = a + c → b = c :=
!add_left_cancel_semigroup.add_left_cancel

abbreviation eq_of_add_eq_add_left := @add.left_cancel

structure add_right_cancel_semigroup [class] (A : Type) extends add_semigroup A :=
(add_right_cancel : ∀a b c, add a b = add c b → a = c)

theorem add.right_cancel [add_right_cancel_semigroup A] {a b c : A} : a + b = c + b → a = c :=
!add_right_cancel_semigroup.add_right_cancel

abbreviation eq_of_add_eq_add_right := @add.right_cancel

/- monoid -/

structure monoid [class] (A : Type) extends semigroup A, has_one A :=
(one_mul : ∀a, mul one a = a) (mul_one : ∀a, mul a one = a)

theorem one_mul [simp] [monoid A] (a : A) : 1 * a = a := !monoid.one_mul

theorem mul_one [simp] [monoid A] (a : A) : a * 1 = a := !monoid.mul_one

structure comm_monoid [class] (A : Type) extends monoid A, comm_semigroup A

/- additive monoid -/

structure add_monoid [class] (A : Type) extends add_semigroup A, has_zero A :=
(zero_add : ∀a, add zero a = a) (add_zero : ∀a, add a zero = a)

theorem zero_add [simp] [add_monoid A] (a : A) : 0 + a = a := !add_monoid.zero_add

theorem add_zero [simp] [add_monoid A] (a : A) : a + 0 = a := !add_monoid.add_zero

structure add_comm_monoid [class] (A : Type) extends add_monoid A, add_comm_semigroup A

definition add_monoid.to_monoid {A : Type} [add_monoid A] : monoid A :=
⦃ monoid,
  mul         := add_monoid.add,
  mul_assoc   := add_monoid.add_assoc,
  one         := add_monoid.zero A,
  mul_one     := add_monoid.add_zero,
  one_mul     := add_monoid.zero_add
⦄

definition add_comm_monoid.to_comm_monoid {A : Type} [add_comm_monoid A] : comm_monoid A :=
⦃ comm_monoid,
  add_monoid.to_monoid,
  mul_comm    := add_comm_monoid.add_comm
⦄

definition monoid.to_add_monoid {A : Type} [s : monoid A] : add_monoid A :=
⦃ add_monoid,
  add         := monoid.mul,
  add_assoc   := monoid.mul_assoc,
  zero        := monoid.one A,
  add_zero    := monoid.mul_one,
  zero_add    := monoid.one_mul
⦄

definition comm_monoid.to_add_comm_monoid {A : Type} [s : comm_monoid A] : add_comm_monoid A :=
⦃ add_comm_monoid,
  monoid.to_add_monoid,
  add_comm    := comm_monoid.mul_comm
⦄

section add_comm_monoid
  variables [add_comm_monoid A]

  theorem add_comm_three  (a b c : A) : a + b + c = c + b + a :=
  by simp

  theorem add.comm4 : ∀ (n m k l : A), n + m + (k + l) = n + k + (m + l) :=
  by simp
end add_comm_monoid

/- group -/

structure group [class] (A : Type) extends monoid A, has_inv A :=
(mul_left_inv : ∀a, mul (inv a) a = one)

-- Note: with more work, we could derive the axiom one_mul

section group
  variable [group A]

  theorem mul.left_inv [simp] (a : A) : a⁻¹ * a = 1 := !group.mul_left_inv

  theorem inv_mul_cancel_left [simp] (a b : A) : a⁻¹ * (a * b) = b :=
  by rewrite [-mul.assoc, mul.left_inv, one_mul]

  theorem inv_mul_cancel_right [simp] (a b : A) : a * b⁻¹ * b = a :=
  by simp

  theorem inv_eq_of_mul_eq_one {a b : A} (H : a * b = 1) : a⁻¹ = b :=
  have a⁻¹ * 1 = b, by inst_simp,
  by inst_simp

  theorem one_inv [simp] : 1⁻¹ = (1 : A) :=
  inv_eq_of_mul_eq_one (one_mul 1)

  theorem inv_inv [simp] (a : A) : (a⁻¹)⁻¹ = a :=
  inv_eq_of_mul_eq_one (mul.left_inv a)

  variable (A)
  theorem left_inverse_inv : function.left_inverse (λ a : A, a⁻¹) (λ a, a⁻¹) :=
  take a, inv_inv a
  variable {A}

  theorem inv.inj {a b : A} (H : a⁻¹ = b⁻¹) : a = b :=
  have a = a⁻¹⁻¹, by simp_nohyps,
  by inst_simp

  theorem inv_eq_inv_iff_eq (a b : A) : a⁻¹ = b⁻¹ ↔ a = b :=
  iff.intro (assume H, inv.inj H) (by simp)

  theorem inv_eq_one_iff_eq_one (a : A) : a⁻¹ = 1 ↔ a = 1 :=
  have a⁻¹ = 1⁻¹ ↔ a = 1, from inv_eq_inv_iff_eq a 1,
  by simp

  theorem eq_one_of_inv_eq_one (a : A) : a⁻¹ = 1 → a = 1 :=
  iff.mp !inv_eq_one_iff_eq_one

  theorem eq_inv_of_eq_inv {a b : A} (H : a = b⁻¹) : b = a⁻¹ :=
  by simp

  theorem eq_inv_iff_eq_inv (a b : A) : a = b⁻¹ ↔ b = a⁻¹ :=
  iff.intro !eq_inv_of_eq_inv !eq_inv_of_eq_inv

  theorem eq_inv_of_mul_eq_one {a b : A} (H : a * b = 1) : a = b⁻¹ :=
  have a⁻¹ = b, from inv_eq_of_mul_eq_one H,
  by inst_simp

  theorem mul.right_inv [simp] (a : A) : a * a⁻¹ = 1 :=
  have a = a⁻¹⁻¹, by simp,
  by inst_simp

  theorem mul_inv_cancel_left [simp] (a b : A) : a * (a⁻¹ * b) = b :=
  by inst_simp

  theorem mul_inv_cancel_right [simp] (a b : A) : a * b * b⁻¹ = a :=
  by inst_simp

  theorem mul_inv [simp] (a b : A) : (a * b)⁻¹ = b⁻¹ * a⁻¹ :=
  inv_eq_of_mul_eq_one (by inst_simp)

  theorem eq_of_mul_inv_eq_one {a b : A} (H : a * b⁻¹ = 1) : a = b :=
  have a⁻¹ * 1 = a⁻¹, by inst_simp,
  by inst_simp

  theorem eq_mul_inv_of_mul_eq {a b c : A} (H : a * c = b) : a = b * c⁻¹ :=
  by simp

  theorem eq_inv_mul_of_mul_eq {a b c : A} (H : b * a = c) : a = b⁻¹ * c :=
  by simp

  theorem inv_mul_eq_of_eq_mul {a b c : A} (H : b = a * c) : a⁻¹ * b = c :=
  by simp

  theorem mul_inv_eq_of_eq_mul {a b c : A} (H : a = c * b) : a * b⁻¹ = c :=
  by simp

  theorem eq_mul_of_mul_inv_eq {a b c : A} (H : a * c⁻¹ = b) : a = b * c :=
  by simp

  theorem eq_mul_of_inv_mul_eq {a b c : A} (H : b⁻¹ * a = c) : a = b * c :=
  by simp

  theorem mul_eq_of_eq_inv_mul {a b c : A} (H : b = a⁻¹ * c) : a * b = c :=
  by simp

  theorem mul_eq_of_eq_mul_inv {a b c : A} (H : a = c * b⁻¹) : a * b = c :=
  by simp

  theorem mul_eq_iff_eq_inv_mul (a b c : A) : a * b = c ↔ b = a⁻¹ * c :=
  iff.intro eq_inv_mul_of_mul_eq mul_eq_of_eq_inv_mul

  theorem mul_eq_iff_eq_mul_inv (a b c : A) : a * b = c ↔ a = c * b⁻¹ :=
  iff.intro eq_mul_inv_of_mul_eq mul_eq_of_eq_mul_inv

  theorem mul_left_cancel {a b c : A} (H : a * b = a * c) : b = c :=
  have a⁻¹ * (a * b) = b, by inst_simp,
  by inst_simp

  theorem mul_right_cancel {a b c : A} (H : a * b = c * b) : a = c :=
  have a * b * b⁻¹ = a, by inst_simp,
  by inst_simp

  theorem mul_eq_one_of_mul_eq_one {a b : A} (H : b * a = 1) : a * b = 1 :=
  by rewrite [-inv_eq_of_mul_eq_one H, mul.left_inv]

  theorem mul_eq_one_iff_mul_eq_one (a b : A) : a * b = 1 ↔ b * a = 1 :=
  iff.intro !mul_eq_one_of_mul_eq_one !mul_eq_one_of_mul_eq_one

  definition conj_by (g a : A) := g * a * g⁻¹
  definition is_conjugate (a b : A) := ∃ x, conj_by x b = a

  local infixl ` ~ ` := is_conjugate
  local infixr ` ∘c `:55 := conj_by

  local attribute conj_by [reducible]

  lemma conj_compose [simp] (f g a : A) : f ∘c g ∘c a = f*g ∘c a :=
  by inst_simp

  lemma conj_id [simp] (a : A) : 1 ∘c a = a :=
  by inst_simp

  lemma conj_one [simp] (g : A) : g ∘c 1 = 1 :=
  by inst_simp

  lemma conj_inv_cancel [simp] (g : A) : ∀ a, g⁻¹ ∘c g ∘c a = a :=
  by inst_simp

  lemma conj_inv [simp] (g : A) : ∀ a, (g ∘c a)⁻¹ = g ∘c a⁻¹ :=
  by inst_simp

  lemma is_conj.refl (a : A) : a ~ a := exists.intro 1 (conj_id a)

  lemma is_conj.symm (a b : A) : a ~ b → b ~ a :=
  assume Pab, obtain x (Pconj : x ∘c b = a), from Pab,
  have Pxinv : x⁻¹ ∘c x ∘c b = x⁻¹ ∘c a,   by simp,
  exists.intro x⁻¹ (by simp)

  lemma is_conj.trans (a b c : A) : a ~ b → b ~ c → a ~ c :=
  assume Pab, assume Pbc,
  obtain x (Px : x ∘c b = a), from Pab,
  obtain y (Py : y ∘c c = b), from Pbc,
  exists.intro (x*y) (by inst_simp)

end group

definition group.to_left_cancel_semigroup [trans_instance] [s : group A] :
    left_cancel_semigroup A :=
⦃ left_cancel_semigroup, s,
  mul_left_cancel := @mul_left_cancel A s ⦄

definition group.to_right_cancel_semigroup [trans_instance] [s : group A] :
    right_cancel_semigroup A :=
⦃ right_cancel_semigroup, s,
  mul_right_cancel := @mul_right_cancel A s ⦄

structure comm_group [class] (A : Type) extends group A, comm_monoid A

/- additive group -/

structure add_group [class] (A : Type) extends add_monoid A, has_neg A :=
(add_left_inv : ∀a, add (neg a) a = zero)

definition add_group.to_group {A : Type} [add_group A] : group A :=
⦃ group, add_monoid.to_monoid,
  mul_left_inv := add_group.add_left_inv ⦄

definition group.to_add_group {A : Type} [s : group A] : add_group A :=
⦃ add_group, monoid.to_add_monoid,
  add_left_inv := group.mul_left_inv ⦄

section add_group
  variables [s : add_group A]
  include s

  theorem add.left_inv [simp] (a : A) : -a + a = 0 := !add_group.add_left_inv

  theorem neg_add_cancel_left [simp] (a b : A) : -a + (a + b) = b :=
  calc -a + (a + b) = (-a + a) + b : by rewrite add.assoc
               ...  = b            : by simp

  theorem neg_add_cancel_right [simp] (a b : A) : a + -b + b = a :=
  by simp

  theorem neg_eq_of_add_eq_zero {a b : A} (H : a + b = 0) : -a = b :=
  have -a + 0 = b, by inst_simp,
  by inst_simp

  theorem neg_zero [simp] : -0 = (0 : A) := neg_eq_of_add_eq_zero (zero_add 0)

  theorem neg_neg [simp] (a : A) : -(-a) = a := neg_eq_of_add_eq_zero (add.left_inv a)

  variable (A)
  theorem left_inverse_neg : function.left_inverse (λ a : A, - a) (λ a, - a) :=
  take a, neg_neg a
  variable {A}

  theorem eq_neg_of_add_eq_zero {a b : A} (H : a + b = 0) : a = -b :=
  have -a = b, from neg_eq_of_add_eq_zero H,
  by inst_simp

  theorem neg.inj {a b : A} (H : -a = -b) : a = b :=
  have a = -(-a), by simp_nohyps,
  by inst_simp

  theorem neg_eq_neg_iff_eq (a b : A) : -a = -b ↔ a = b :=
  iff.intro (assume H, neg.inj H) (by simp)

  theorem eq_of_neg_eq_neg {a b : A} : -a = -b → a = b :=
  iff.mp !neg_eq_neg_iff_eq

  theorem neg_eq_zero_iff_eq_zero (a : A) : -a = 0 ↔ a = 0 :=
  have -a = -0 ↔ a = 0, from neg_eq_neg_iff_eq a 0,
  by simp

  theorem eq_zero_of_neg_eq_zero {a : A} : -a = 0 → a = 0 :=
  iff.mp !neg_eq_zero_iff_eq_zero

  theorem eq_neg_of_eq_neg {a b : A} (H : a = -b) : b = -a :=
  by simp

  theorem eq_neg_iff_eq_neg (a b : A) : a = -b ↔ b = -a :=
  iff.intro !eq_neg_of_eq_neg !eq_neg_of_eq_neg

  theorem add.right_inv [simp] (a : A) : a + -a = 0 :=
  have a = -(-a), by simp,
  by inst_simp

  theorem add_neg_cancel_left [simp] (a b : A) : a + (-a + b) = b :=
  by inst_simp

  theorem add_neg_cancel_right [simp] (a b : A) : a + b + -b = a :=
  by simp

  theorem neg_add_rev [simp] (a b : A) : -(a + b) = -b + -a :=
  neg_eq_of_add_eq_zero (by simp)

  -- TODO: delete these in favor of sub rules?
  theorem eq_add_neg_of_add_eq {a b c : A} (H : a + c = b) : a = b + -c :=
  by simp

  theorem eq_neg_add_of_add_eq {a b c : A} (H : b + a = c) : a = -b + c :=
  by simp

  theorem neg_add_eq_of_eq_add {a b c : A} (H : b = a + c) : -a + b = c :=
  by simp

  theorem add_neg_eq_of_eq_add {a b c : A} (H : a = c + b) : a + -b = c :=
  by simp

  theorem eq_add_of_add_neg_eq {a b c : A} (H : a + -c = b) : a = b + c :=
  by simp

  theorem eq_add_of_neg_add_eq {a b c : A} (H : -b + a = c) : a = b + c :=
  by simp

  theorem add_eq_of_eq_neg_add {a b c : A} (H : b = -a + c) : a + b = c :=
  by simp

  theorem add_eq_of_eq_add_neg {a b c : A} (H : a = c + -b) : a + b = c :=
  by simp

  theorem add_eq_iff_eq_neg_add (a b c : A) : a + b = c ↔ b = -a + c :=
  iff.intro eq_neg_add_of_add_eq add_eq_of_eq_neg_add

  theorem add_eq_iff_eq_add_neg (a b c : A) : a + b = c ↔ a = c + -b :=
  iff.intro eq_add_neg_of_add_eq add_eq_of_eq_add_neg

  theorem add_left_cancel {a b c : A} (H : a + b = a + c) : b = c :=
  have -a + (a + b) = b, by inst_simp,
  by inst_simp

  theorem add_right_cancel {a b c : A} (H : a + b = c + b) : a = c :=
  have a + b + -b = a, by inst_simp,
  by inst_simp

  definition add_group.to_add_left_cancel_semigroup [trans_instance] :
    add_left_cancel_semigroup A :=
  ⦃ add_left_cancel_semigroup, s,
    add_left_cancel := @add_left_cancel A s ⦄

  definition add_group.to_add_right_cancel_semigroup [trans_instance] :
    add_right_cancel_semigroup A :=
  ⦃ add_right_cancel_semigroup, s,
    add_right_cancel := @add_right_cancel A s ⦄

  theorem add_neg_eq_neg_add_rev {a b : A} : a + -b = -(b + -a) :=
  by simp

  theorem ne_add_of_ne_zero_right (a : A) {b : A} (H : b ≠ 0) : a ≠ b + a :=
    begin
      intro Heq,
      apply H,
      rewrite [-zero_add a at Heq{1}],
      let Heq' := eq_of_add_eq_add_right Heq,
      apply eq.symm Heq'
    end

  theorem ne_add_of_ne_zero_left (a : A) {b : A} (H : b ≠ 0) : a ≠ a + b :=
    begin
      intro Heq,
      apply H,
      rewrite [-add_zero a at Heq{1}],
      let Heq' := eq_of_add_eq_add_left Heq,
      apply eq.symm Heq'
    end

  /- sub -/

  -- TODO: derive corresponding facts for div in a field
  protected definition algebra.sub [reducible] (a b : A) : A := a + -b

  definition add_group_has_sub [instance] : has_sub A :=
  has_sub.mk algebra.sub

  theorem sub_eq_add_neg [simp] (a b : A) : a - b = a + -b := rfl

  theorem sub_self (a : A) : a - a = 0 := !add.right_inv

  theorem sub_add_cancel (a b : A) : a - b + b = a := !neg_add_cancel_right

  theorem add_sub_cancel (a b : A) : a + b - b = a := !add_neg_cancel_right

  theorem add_sub_assoc (a b c : A) : a + b - c = a + (b - c) :=
    by rewrite [sub_eq_add_neg, add.assoc, -sub_eq_add_neg]

  theorem eq_of_sub_eq_zero {a b : A} (H : a - b = 0) : a = b :=
  have -a + 0 = -a, by inst_simp,
  by inst_simp

  theorem eq_iff_sub_eq_zero (a b : A) : a = b ↔ a - b = 0 :=
  iff.intro (assume H, eq.subst H !sub_self) (assume H, eq_of_sub_eq_zero H)

  theorem zero_sub (a : A) : 0 - a = -a := !zero_add

  theorem sub_zero (a : A) : a - 0 = a :=
  by simp

  theorem sub_ne_zero_of_ne {a b : A} (H : a ≠ b) : a - b ≠ 0 :=
    begin
      intro Hab,
      apply H,
      apply eq_of_sub_eq_zero Hab
    end

  theorem sub_neg_eq_add (a b : A) : a - (-b) = a + b :=
  by simp

  theorem neg_sub (a b : A) : -(a - b) = b - a :=
  neg_eq_of_add_eq_zero (by inst_simp)

  theorem add_sub (a b c : A) : a + (b - c) = a + b - c :=
  by simp

  theorem sub_add_eq_sub_sub_swap (a b c : A) : a - (b + c) = a - c - b :=
  by inst_simp

  theorem sub_eq_iff_eq_add (a b c : A) : a - b = c ↔ a = c + b :=
  iff.intro (assume H, eq_add_of_add_neg_eq H) (assume H, add_neg_eq_of_eq_add H)

  theorem eq_sub_iff_add_eq (a b c : A) : a = b - c ↔ a + c = b :=
  iff.intro (assume H, add_eq_of_eq_add_neg H) (assume H, eq_add_neg_of_add_eq H)

  theorem eq_iff_eq_of_sub_eq_sub {a b c d : A} (H : a - b = c - d) : a = b ↔ c = d :=
  calc
    a = b ↔ a - b = 0   : eq_iff_sub_eq_zero
      ... = (c - d = 0) : H
      ... ↔ c = d       : iff.symm (eq_iff_sub_eq_zero c d)

  theorem eq_sub_of_add_eq {a b c : A} (H : a + c = b) : a = b - c :=
  by simp

  theorem sub_eq_of_eq_add {a b c : A} (H : a = c + b) : a - b = c :=
  by simp

  theorem eq_add_of_sub_eq {a b c : A} (H : a - c = b) : a = b + c :=
  by simp

  theorem add_eq_of_eq_sub {a b c : A} (H : a = c - b) : a + b = c :=
  by simp

  theorem left_inverse_sub_add_left (c : A) : function.left_inverse (λ x, x - c) (λ x, x + c) :=
  take x, add_sub_cancel x c

  theorem left_inverse_add_left_sub (c : A) : function.left_inverse (λ x, x + c) (λ x, x - c) :=
  take x, sub_add_cancel x c

  theorem left_inverse_add_right_neg_add (c : A) :
      function.left_inverse (λ x, c + x) (λ x, - c + x) :=
  take x, add_neg_cancel_left c x

  theorem left_inverse_neg_add_add_right (c : A) :
      function.left_inverse (λ x, - c + x) (λ x, c + x) :=
  take x, neg_add_cancel_left c x
end add_group

structure add_comm_group [class] (A : Type) extends add_group A, add_comm_monoid A

definition add_comm_group.to_comm_group (A : Type) [s : add_comm_group A] : comm_group A :=
⦃ comm_group, add_group.to_group,
  mul_comm := add_comm_group.add_comm ⦄

definition comm_group.to_add_comm_group (A : Type) [s : comm_group A] : add_comm_group A :=
⦃ add_comm_group, group.to_add_group,
  add_comm := comm_group.mul_comm ⦄


section add_comm_group
  variable [s : add_comm_group A]
  include s

  theorem sub_add_eq_sub_sub (a b c : A) : a - (b + c) = a - b - c :=
  by simp

  theorem neg_add_eq_sub (a b : A) : -a + b = b - a :=
  by simp

  theorem neg_add (a b : A) : -(a + b) = -a + -b :=
  by simp

  theorem sub_add_eq_add_sub (a b c : A) : a - b + c = a + c - b :=
  by simp

  theorem sub_sub (a b c : A) : a - b - c = a - (b + c) :=
  by simp

  theorem add_sub_add_left_eq_sub (a b c : A) : (c + a) - (c + b) = a - b :=
  by simp

  theorem eq_sub_of_add_eq' {a b c : A} (H : c + a = b) : a = b - c :=
  by simp

  theorem sub_eq_of_eq_add' {a b c : A} (H : a = b + c) : a - b = c :=
  by simp

  theorem eq_add_of_sub_eq' {a b c : A} (H : a - b = c) : a = b + c :=
  by simp

  theorem add_eq_of_eq_sub' {a b c : A} (H : b = c - a) : a + b = c :=
  by simp

  theorem sub_sub_self (a b : A) : a - (a - b) = b :=
  by simp

  theorem add_sub_comm (a b c d : A) : a + b - (c + d) = (a - c) + (b - d) :=
  by simp

  theorem sub_eq_sub_add_sub (a b c : A) : a - b = c - b + (a - c) :=
  by simp

  theorem neg_neg_sub_neg (a b : A) : - (-a - -b) = a - b :=
  by simp

end add_comm_group

definition group_of_add_group (A : Type) [G : add_group A] : group A :=
⦃group,
  mul             := has_add.add,
  mul_assoc       := add.assoc,
  one             := !has_zero.zero,
  one_mul         := zero_add,
  mul_one         := add_zero,
  inv             := has_neg.neg,
  mul_left_inv    := add.left_inv⦄

namespace norm_num
reveal add.assoc

definition add1 [has_add A] [has_one A] (a : A) : A := add a one

local attribute add1 bit0 bit1 [reducible]

theorem add_comm_four [add_comm_semigroup A] (a b : A) : a + a + (b + b) = (a + b) + (a + b) :=
by simp

theorem add_comm_middle [add_comm_semigroup A] (a b c : A) : a + b + c = a + c + b :=
by simp

theorem bit0_add_bit0 [add_comm_semigroup A] (a b : A) : bit0 a + bit0 b = bit0 (a + b) :=
by simp

theorem bit0_add_bit0_helper [add_comm_semigroup A] (a b t : A) (H : a + b = t) :
        bit0 a + bit0 b = bit0 t :=
by rewrite -H; simp

theorem bit1_add_bit0 [add_comm_semigroup A] [has_one A] (a b : A) :
        bit1 a + bit0 b = bit1 (a + b) :=
by simp

theorem bit1_add_bit0_helper [add_comm_semigroup A] [has_one A] (a b t : A)
        (H : a + b = t) : bit1 a + bit0 b = bit1 t :=
by rewrite -H; simp

theorem bit0_add_bit1 [add_comm_semigroup A] [has_one A] (a b : A) :
        bit0 a + bit1 b = bit1 (a + b) :=
by simp

theorem bit0_add_bit1_helper [add_comm_semigroup A] [has_one A] (a b t : A)
        (H : a + b = t) : bit0 a + bit1 b = bit1 t :=
by rewrite -H; simp

theorem bit1_add_bit1 [add_comm_semigroup A] [has_one A] (a b : A) :
        bit1 a + bit1 b = bit0 (add1 (a + b)) :=
by simp

theorem bit1_add_bit1_helper [add_comm_semigroup A] [has_one A] (a b t s: A)
        (H : (a + b) = t) (H2 : add1 t = s) : bit1 a + bit1 b = bit0 s :=
by inst_simp

theorem bin_add_zero [add_monoid A] (a : A) : a + zero = a :=
by simp

theorem bin_zero_add [add_monoid A] (a : A) : zero + a = a :=
by simp

theorem one_add_bit0 [add_comm_semigroup A] [has_one A] (a : A) : one + bit0 a = bit1 a :=
by simp

theorem bit0_add_one [has_add A] [has_one A] (a : A) : bit0 a + one = bit1 a :=
rfl

theorem bit1_add_one [has_add A] [has_one A] (a : A) : bit1 a + one = add1 (bit1 a) :=
rfl

theorem bit1_add_one_helper [has_add A] [has_one A] (a t : A) (H : add1 (bit1 a) = t) :
        bit1 a + one = t :=
by inst_simp

theorem one_add_bit1 [add_comm_semigroup A] [has_one A] (a : A) : one + bit1 a = add1 (bit1 a) :=
by simp

theorem one_add_bit1_helper [add_comm_semigroup A] [has_one A] (a t : A)
        (H : add1 (bit1 a) = t) : one + bit1 a = t :=
by inst_simp

theorem add1_bit0 [has_add A] [has_one A] (a : A) : add1 (bit0 a) = bit1 a :=
rfl

theorem add1_bit1 [add_comm_semigroup A] [has_one A] (a : A) :
        add1 (bit1 a) = bit0 (add1 a) :=
by simp

theorem add1_bit1_helper [add_comm_semigroup A] [has_one A] (a t : A) (H : add1 a = t) :
        add1 (bit1 a) = bit0 t :=
by inst_simp

theorem add1_one [has_add A] [has_one A] : add1 (one : A) = bit0 one :=
rfl

theorem add1_zero [add_monoid A] [has_one A] : add1 (zero : A) = one :=
by simp

theorem one_add_one [has_add A] [has_one A] : (one : A) + one = bit0 one :=
rfl

theorem subst_into_sum [has_add A] (l r tl tr t : A) (prl : l = tl) (prr : r = tr)
        (prt : tl + tr = t) : l + r = t :=
by simp

theorem neg_zero_helper [add_group A] (a : A) (H : a = 0) : - a = 0 :=
by simp

end norm_num

attribute [simp]
  zero_add add_zero one_mul mul_one
  at simplifier.unit

attribute [simp]
  neg_neg sub_eq_add_neg
  at simplifier.neg

attribute [simp]
  add.assoc add.comm add.left_comm
  mul.left_comm mul.comm mul.assoc
  at simplifier.ac