∀ {σ : Type u_1} {R : Type u_2} {S : Type u_3} [inst : DecidableEq σ] [inst_1 : CommSemiring R]
  [inst_2 : CommSemiring S] (n : σ →₀ ℕ) (f : R →+* S) (p : MvPowerSeries σ R),
  (MvPowerSeries.trunc' S n) ((MvPowerSeries.map f) p) = (MvPolynomial.map f) ((MvPowerSeries.trunc' R n) p)