∀ {σ : Type u_1} {R : Type u_3} {S : Type u_4} [inst : CommSemiring R] [inst_1 : CommSemiring S]
  (f : R →+* MvPolynomial σ S) (φ : MvPolynomial σ R),
  MvPolynomial.join₂ ((MvPolynomial.map f) φ) = (MvPolynomial.bind₂ f) φ