(* Compatibility *)
(* (comp U V) iff (unmark U)=(unmark V) *)
Hint Resolve Comp_Var Comp_Fun Comp_Ap.

(* Specifications: *)
Lemma comp_refl : forall U : redexes, comp U U.
Lemma comp_sym : forall U V : redexes, comp U V -> comp V U.
Lemma comp_trans :
forall U V : redexes,
comp U V -> forall (W : redexes) (CVW : comp V W), comp U W.