∀ {A : Type u₁} [inst : CategoryTheory.Category.{v₁, u₁} A] {B : Type u₂} [inst_1 : CategoryTheory.Category.{v₂, u₂} B]
  {T : Type u₃} [inst_2 : CategoryTheory.Category.{v₃, u₃} T] (R : CategoryTheory.Functor B T)
  {L₁ L₂ : CategoryTheory.Functor A T} (i : L₁ ≅ L₂) (X : CategoryTheory.Comma L₁ R),
  ((CategoryTheory.Comma.mapLeftIso R i).functor.obj X).hom =
    CategoryTheory.CategoryStruct.comp (i.inv.app X.left) X.hom