(** If two angles ACB and ADB are inscribed in the same circle, then they are either congruent or supplementary. *)

Lemma cop2_onc4__or_conga_suppa : forall O P A B C C',
A <> B -> B <> C -> A <> C -> B <> C' -> A <> C' ->
OnCircle A O P -> OnCircle B O P -> OnCircle C O P -> OnCircle C' O P ->
Coplanar A B C O -> Coplanar A B C' O ->
CongA A C B A C' B \/ SuppA A C B A C' B.