∀ {C : Type u_2} [inst : CategoryTheory.Category.{u_1, u_2} C] {Q R S : C} {h : Q ⟶ S} {k : R ⟶ S}
  (hr : CategoryTheory.RegularMono h)
  (s :
    CategoryTheory.Limits.Fork (CategoryTheory.CategoryStruct.comp k hr.left)
      (CategoryTheory.CategoryStruct.comp k hr.right)),
  CategoryTheory.CategoryStruct.comp (CategoryTheory.CategoryStruct.comp s.ι k) hr.left =
    CategoryTheory.CategoryStruct.comp (CategoryTheory.CategoryStruct.comp s.ι k) hr.right