∀ {C : Type u₁} [inst : CategoryTheory.Category.{v₁, u₁} C] {D : Type u₂} [inst_1 : CategoryTheory.Category.{v₂, u₂} D]
  {F G H : CategoryTheory.Functor C Dᵒᵖ} (α : F ⟶ G) (β : G ⟶ H),
  CategoryTheory.NatTrans.leftOp (CategoryTheory.CategoryStruct.comp α β) =
    CategoryTheory.CategoryStruct.comp (CategoryTheory.NatTrans.leftOp β) (CategoryTheory.NatTrans.leftOp α)