∀ {A : Type u_1} [inst : CategoryTheory.Category.{v_1, u_1} A] {A' : Type u_2}
  [inst_1 : CategoryTheory.Category.{v_2, u_2} A'] {B : Type u} [inst_2 : CategoryTheory.Category.{v, u} B]
  {F : CategoryTheory.Functor (A ⊕ A') B},
  CategoryTheory.Sum.natTransOfWhiskerLeftInlInr
      (CategoryTheory.CategoryStruct.id ((CategoryTheory.Sum.inl_ A A').comp F))
      (CategoryTheory.CategoryStruct.id ((CategoryTheory.Sum.inr_ A A').comp F)) =
    CategoryTheory.CategoryStruct.id F