intro (b₁——b₂)
ℕ : Type
inst✝ : Natural ℕ
c : Difference ℕ
a₁ a₂ : ℕ
⊢ ∀ {b : Difference ℕ}, a₁——a₂ ≃ b → b ≃ c → a₁——a₂ ≃ c
ℕ : Type
inst✝ : Natural ℕ
c : Difference ℕ
a₁ a₂ b₁ b₂ : ℕ
⊢ a₁——a₂ ≃ b₁——b₂ → b₁——b₂ ≃ c → a₁——a₂ ≃ c