proposition: Lemma incl_soustr_in : forall (X:Ensemble U) (x:U), In U X x -> Included U (Subtract U X x) X.
proof: Proof. intros X x H'; red. intros x0 H'0; elim H'0; auto with sets. Qed.
imports: Ensembles Constructive_sets Relations_1 Relations_1_facts Partial_Order Cpo Powerset Powerset_facts Classical Classical_sets