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.Global Instance same_side_morphism : Proper(eq ==> eq ==> EqL ==> iff) same_side.

Lemma th17: forall X Y Z1 Z2 I, ~ ColH X Y Z1 -> ~ ColH X Y Z2 -> ColH X I Y -> BetH Z1 I Z2 -> CongH X Z1 X Z2 -> CongH Y Z1 Y Z2 -> CongaH X Y Z1 X Y Z2.