∀ (C : Type u_2) [inst : CategoryTheory.Category.{u_4, u_2} C] (D : Type u_1)
  [inst_1 : CategoryTheory.Category.{u_3, u_1} D] {X Y Z : CategoryTheory.Core (CategoryTheory.Functor C D)} (f : X ⟶ Y)
  (g : Y ⟶ Z),
  (CategoryTheory.CategoryStruct.comp f g).iso.core.hom =
    CategoryTheory.CategoryStruct.comp f.iso.core.hom g.iso.core.hom