∀ {R : Type u_2} [inst : CommRing R] {τ : Type u_3} {S : Type u_4} [inst_1 : CommRing S] [inst_2 : Algebra R S]
  {a : MvPowerSeries τ S},
  PowerSeries.HasSubst a →
    ∀ (f : PowerSeries R),
      MvPowerSeries.constantCoeff (PowerSeries.subst a f) =
        ∑ᶠ (d : ℕ), (PowerSeries.coeff d) f • MvPowerSeries.constantCoeff (a ^ d)