have : n + m₂ ≃ n + m₁ := AA.substR (Rel.symm ‹m₁ ≃ m₂›)
ℕ : Type
inst✝ : Natural ℕ
n m₁ m₂ : ℕ
x✝ : m₁ ≃ m₂
⊢ n——m₁ ≃ n——m₂
ℕ : Type
inst✝ : Natural ℕ
n m₁ m₂ : ℕ
x✝ : m₁ ≃ m₂
this : n + m₂ ≃ n + m₁
⊢ n——m₁ ≃ n——m₂