∀ {R : Type u} {S : Type v} {T : Type w} [inst : Semiring R] [inst_1 : Semiring S] [inst_2 : Semiring T] (f : S →+* T)
  (g : R →+* S), (Polynomial.mapRingHom f).comp (Polynomial.mapRingHom g) = Polynomial.mapRingHom (f.comp g)