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 congaH_sym : forall A B C D E F, ~ColH A B C -> ~ColH D E F
-> CongaH A B C D E F -> CongaH D E F A B C.