∀ {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} (S R : CategoryTheory.Sieve X),
  CategoryTheory.Sieve.functorPushforward F (S ⊔ R) =
    CategoryTheory.Sieve.functorPushforward F S ⊔ CategoryTheory.Sieve.functorPushforward F R