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