∀ {C : Type u_2} [inst : CategoryTheory.Category.{u_1, u_2} C] {X Y : C} (f : X ⟶ Y)
  [inst_1 : CategoryTheory.Limits.HasPullbacksAlong f] (x : CategoryTheory.Over X) (y : CategoryTheory.Over Y)
  (v : x ⟶ (CategoryTheory.Over.pullback f).obj y),
  CategoryTheory.CategoryStruct.comp
      (CategoryTheory.CategoryStruct.comp v.left (CategoryTheory.Limits.pullback.fst y.hom f)) y.hom =
    ((CategoryTheory.Over.map f).obj x).hom