∀ {B : Type u} [inst : CategoryTheory.Bicategory B] {a b : B} (f : a ⟶ b) {a_1 b_1 : Bᵒᵖ} (g : a_1 ⟶ b_1)
  (X : ↑(CategoryTheory.Cat.of (Opposite.unop a_1 ⟶ a))),
  ((CategoryTheory.Bicategory.postcomp₂ f).naturality g).inv.toNatTrans.app X =
    (CategoryTheory.Bicategory.associator g.unop X f).inv