∀ {T : Type u_6} [inst : CategoryTheory.Category.{u_5, u_6} T] {D : Type u_4}
  [inst_1 : CategoryTheory.Category.{u_3, u_4} D] {C : Type u_2} [inst_2 : CategoryTheory.Category.{u_1, u_2} C]
  (F : CategoryTheory.Functor C T) (G : CategoryTheory.Functor D T) (c : C)
  {Y Z : CategoryTheory.Comma ((CategoryTheory.Under.forget c).comp F) G} (x : Y ⟶ Z),
  CategoryTheory.CategoryStruct.comp ((fun Y => Y.left.hom) Y)
      ((CategoryTheory.Comma.fst F G).map ((CategoryTheory.Comma.preLeft (CategoryTheory.Under.forget c) F G).map x)) =
    (fun Y => Y.left.hom) Z