ext z
d a₁ b₁ a₂ b₂ : ℕ
⊢ pair' (of_fraction d (a₁ + a₂) (b₁ + b₂)) = pair' (of_fraction d a₁ b₁) + pair' (of_fraction d a₂ b₂)
case h
d a₁ b₁ a₂ b₂ : ℕ
z : Fin (Nat.succ 2) → ℤ
⊢ pair' (of_fraction d (a₁ + a₂) (b₁ + b₂)) z = (pair' (of_fraction d a₁ b₁) + pair' (of_fraction d a₂ b₂)) z