Definition cut' A B X Y := X <> Y /\ forall l, IncidL X l -> IncidL Y l -> cut l A B.
Global Instance cut_morphism : Proper(EqL ==> eq ==> eq ==> iff) cut.

Lemma par__or_eq_para : forall A B C D l m,
IncidL A l -> IncidL B l -> IncidL C m -> IncidL D m -> Par A B C D -> Para l m \/ EqL l m.