∀ {R : Type u} {σ : Type u_1} [inst : CommSemiring R] (f : MvPolynomial σ R →+* MvPolynomial σ R),
  f.comp MvPolynomial.C = MvPolynomial.C →
    (∀ (n : σ), f (MvPolynomial.X n) = MvPolynomial.X n) → ∀ (p : MvPolynomial σ R), f p = p