∀ {C : Type u₁} [inst : CategoryTheory.Category.{v₁, u₁} C] {D : Type u₂} [inst_1 : CategoryTheory.Category.{v₂, u₂} D]
  {S S' : D} {T : CategoryTheory.Functor C D} (i : S ≅ S')
  (X : CategoryTheory.Comma (CategoryTheory.Functor.fromPUnit S) T),
  ((CategoryTheory.StructuredArrow.mapIso i).functor.obj X).hom = CategoryTheory.CategoryStruct.comp i.inv X.hom