Work around bug 3422 in trunk

https://coq.inria.fr/bugs/show_bug.cgi?id=3422

Current error:

Unable to unify
 "{|
  object := Functor (F w) (F z);
  morphism := NaturalTransformation (D:=F z);
  compose := composeT (D:=F z);
  trunc_morphism := fun s d : Functor (F w) (F z) =>
                    trunc_natural_transformation s d |}" with
 "{|
  object := Functor (F w) (F z);
  morphism := NaturalTransformation (D:=F z);
  compose := composeT (D:=F z);
  trunc_morphism := fun s d : Functor (F w) (F z) =>
                    trunc_natural_transformation s d |}"