Definition Rel_unfmap : forall A B (f:A->B) x y,
    Rel B (fmap f x) y ->
      exists y', Rel A x y' /\ fmap f y' = y.

(* Specifications: *)
Lemma Rel_refl : forall A x, Rel A x x.
Lemma Rel_trans : forall A x y z, Rel A x y -> Rel A y z -> Rel A x z.