intros.
icase a;
exists CEmpty;
constructor...
Qed.

(* Specifications: *)
Lemma combineJ_self' {DA: Disj_alg A}:
forall a b : combiner, join a a b -> identity a.
Lemma combineJ_self {DA: Disj_alg A}:
forall a b : combiner, join a a b -> a = b.