Theorem: Lean4Axiomatic.Integer.neg_one_mul_neg_one_eqv_one (Lean4Axiomatic/Integer/Sign.lean)

State before:
ℕ : Type
inst✝⁴ : Natural ℕ
ℤ : Type
inst✝³ : Core ℤ
inst✝² : Addition ℤ
inst✝¹ : Multiplication ℤ
inst✝ : Negation ℤ
⊢ -1 * -1 ≃ 1
Tactic:
calc
  (-1 : ℤ) * (-1) ≃ _ := Rel.symm AA.scompatL
  (-(1 * (-1)))   ≃ _ := AA.subst₁ (Rel.symm AA.scompatR)
  (-(-(1 * 1)))   ≃ _ := neg_involutive
  1 * 1           ≃ _ := one_mul_one_eqv_one
  1               ≃ _ := Rel.refl
State after:
no goals