∀ {C : Type u_4} [inst : CategoryTheory.Category.{u_3, u_4} C] {D : Type u_2}
  [inst_1 : CategoryTheory.Category.{u_1, u_2} D] {F G : CategoryTheory.Functor C D} (τ : F ⟶ G)
  (sq : CategoryTheory.Square C),
  CategoryTheory.CategoryStruct.comp (F.mapSquare.obj sq).f₁₂ (τ.app sq.X₂) =
    CategoryTheory.CategoryStruct.comp (τ.app sq.X₁) (G.mapSquare.obj sq).f₁₂