∀ {B : Type u} [inst : CategoryTheory.Bicategory B] {x y z t : B} {f f' : x ⟶ y} {g g' : y ⟶ z} {h h' : z ⟶ t}
  (ef : f = f') (eg : g = g') (eh : h = h'),
  (CategoryTheory.Bicategory.associator f g h).hom =
    CategoryTheory.CategoryStruct.comp (CategoryTheory.eqToHom ⋯)
      (CategoryTheory.CategoryStruct.comp (CategoryTheory.Bicategory.associator f' g' h').hom
        (CategoryTheory.eqToHom ⋯))