∀ {R : Type u_1} {S : Type u_2} {A : Type u_3} [inst : Field R] [inst_1 : Ring A] [inst_2 : Algebra R A]
  [inst_3 : SetLike S A] [hSA : NonUnitalSubringClass S A] [hSRA : SMulMemClass S R A] (s : S) (h1 : 1 ∉ s)
  (a : Unitization R ↥s), ↑((NonUnitalSubalgebra.unitizationAlgEquiv s h1) a) = (algebraMap R A) a.fst + ↑a.snd