(* Disj_alg: adds the property that no nonempty element can join with itself. *)
Class Disj_alg  (t: Type) {J: Join t} :=
join_self: forall {a b}, join a a b -> identity a.
Arguments Disj_alg _ [J].

Lemma identity_core  {A}{J: Join A}{PA: Perm_alg A}{SA: Sep_alg A}:
forall a, identity a -> a = core a.