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 OS2__TS : forall O X Y Z,
same_side' Y Z O X -> same_side' X Y O Z -> cut' X Z O Y.