cases b
case mk
b c : ℤα
z✝ w✝ : ℤ
⊢ mul (mul { z := z✝, w := w✝ } b) c = mul { z := z✝, w := w✝ } (mul b c)
case mk.mk
c : ℤα
z✝¹ w✝¹ z✝ w✝ : ℤ
⊢ mul (mul { z := z✝¹, w := w✝¹ } { z := z✝, w := w✝ }) c = mul { z := z✝¹, w := w✝¹ } (mul { z := z✝, w := w✝ } c)