∀ (R : Type u_1) [inst : CommRing R] (n m k : ℕ) (hn : n = m + k)
  (f : TensorProduct R (MvPolynomial (Fin m) R) (MvPolynomial (Fin k) R)),
  (MvPolynomial.universalFactorizationMapPresentation R n m k hn).σ' f =
    (MvPolynomial.map MvPolynomial.C) ((MvPolynomial.tensorEquivSum R (Fin m) (Fin k) R) f)