∀ {R : Type u_2} [inst : CommRing R] {S : Type u_4} [inst_1 : CommRing S] {υ : Type u_5} {T : Type u_6}
  [inst_2 : CommRing T] [inst_3 : Algebra R S] [inst_4 : Algebra R T] [inst_5 : Algebra S T] {a : PowerSeries S}
  {b : MvPowerSeries υ T} [IsScalarTower R S T],
  PowerSeries.HasSubst a →
    PowerSeries.HasSubst b → PowerSeries.subst b ∘ PowerSeries.subst a = PowerSeries.subst (PowerSeries.subst b a)