(** If the angles ACB and ADB are congruent and inscribed in the same circle, then C and D lie on the same side of AB. *)

(* Specifications: *)
Lemma conga_cop2_onc4__os : forall O P A B C D, ~ Col A B O ->
OnCircle A O P -> OnCircle B O P -> OnCircle C O P -> OnCircle D O P ->
Coplanar A B O C -> Coplanar A B O D -> CongA A C B A D B ->
OS A B C D.