∀ {σ : Type u_1} {R : Type u_3} [inst : CommRing R] {τ : Type u_4} {S : Type u_5} [inst_1 : CommRing S]
  [inst_2 : Algebra R S] {a : σ → MvPowerSeries τ S},
  MvPowerSeries.HasSubst a →
    ∀ (f : MvPowerSeries σ R),
      MvPowerSeries.constantCoeff (MvPowerSeries.subst a f) =
        ∑ᶠ (d : σ →₀ ℕ), (MvPowerSeries.coeff d) f • MvPowerSeries.constantCoeff (d.prod fun s e => a s ^ e)