∀ {R : Type u} {S : Type v} {ι : Type w} [inst : CommRing R] [inst_1 : CommRing S] [inst_2 : Algebra R S]
  {ι' : Type u_1} {T : Type u_2} [inst_3 : CommRing T] [inst_4 : Algebra R T] [inst_5 : Algebra S T]
  [inst_6 : IsScalarTower R S T] (Q : Algebra.Generators S T ι') (P : Algebra.Generators R S ι) (a : ι' ⊕ ι),
  (Q.comp P).val a = Sum.elim Q.val (⇑(algebraMap S T) ∘ P.val) a