∀ {C : Type u₁} [inst : CategoryTheory.Category.{v₁, u₁} C] {D : Type u₂} [inst_1 : CategoryTheory.Category.{v₂, u₂} D]
  (F : CategoryTheory.Functor C D) {X : C} {E : Type u₃} [inst_2 : CategoryTheory.Category.{v₃, u₃} E]
  (G : CategoryTheory.Functor D E) (R : CategoryTheory.Sieve ((F.comp G).obj X)),
  CategoryTheory.Sieve.functorPullback (F.comp G) R =
    CategoryTheory.Sieve.functorPullback F (CategoryTheory.Sieve.functorPullback G R)