∀ {B : Type u_3} [inst : CategoryTheory.Bicategory B] {a b : B} {f : a ⟶ b}
  {t : CategoryTheory.Bicategory.LeftExtension f (CategoryTheory.CategoryStruct.id a)} (H' : (t.whisker f).IsKan),
  CategoryTheory.Bicategory.leftZigzag t.unit
      (H'.desc
        (CategoryTheory.Bicategory.LeftExtension.mk (CategoryTheory.CategoryStruct.id b)
          (CategoryTheory.CategoryStruct.comp (CategoryTheory.Bicategory.leftUnitor f).hom
            (CategoryTheory.Bicategory.rightUnitor f).inv))) =
    CategoryTheory.CategoryStruct.comp (CategoryTheory.Bicategory.leftUnitor f).hom
      (CategoryTheory.Bicategory.rightUnitor f).inv