simp only [add, add_assoc]
case mk.mk.mk
z✝² w✝² z✝¹ w✝¹ z✝ w✝ : ℤ
⊢ add (add { z := z✝², w := w✝² } { z := z✝¹, w := w✝¹ }) { z := z✝, w := w✝ } =
    add { z := z✝², w := w✝² } (add { z := z✝¹, w := w✝¹ } { z := z✝, w := w✝ })
no goals