∀ {B : Type u} [inst : CategoryTheory.Bicategory B] {a b c : B} {f : a ⟶ b} {g : a ⟶ c}
  [inst_1 : CategoryTheory.Bicategory.HasLeftKanExtension f g] (s : CategoryTheory.Bicategory.LeftExtension f g)
  {Z : a ⟶ c} (h : CategoryTheory.CategoryStruct.comp f s.extension ⟶ Z),
  CategoryTheory.CategoryStruct.comp (CategoryTheory.Bicategory.lanUnit f g)
      (CategoryTheory.CategoryStruct.comp
        (CategoryTheory.Bicategory.whiskerLeft f (CategoryTheory.Bicategory.lanDesc s)) h) =
    CategoryTheory.CategoryStruct.comp s.unit h