∀ {B : Type u} [self : CategoryTheory.Bicategory B] {a b : B} {f g : a ⟶ b} (η : f ⟶ g),
  CategoryTheory.Bicategory.whiskerLeft (CategoryTheory.CategoryStruct.id a) η =
    CategoryTheory.CategoryStruct.comp (CategoryTheory.Bicategory.leftUnitor f).hom
      (CategoryTheory.CategoryStruct.comp η (CategoryTheory.Bicategory.leftUnitor g).inv)