∀ {B : Type u_2} [inst : CategoryTheory.Bicategory B] (a b : B) {X Y : a ⟶ b} (f : X ⟶ Y),
  CategoryTheory.CategoryStruct.comp
      (((CategoryTheory.Bicategory.postcomposing a b b).obj (CategoryTheory.CategoryStruct.id b)).map f)
      ((fun x => CategoryTheory.Bicategory.rightUnitor x) Y).hom =
    CategoryTheory.CategoryStruct.comp ((fun x => CategoryTheory.Bicategory.rightUnitor x) X).hom
      ((CategoryTheory.Functor.id (a ⟶ b)).map f)