(** Euclid Book III Prop 21: In a circle the angles in the same segment are equal to one another. *)

Lemma cop_onc4_os__conga : forall O P A B C C',
OnCircle A O P -> OnCircle B O P -> OnCircle C O P -> OnCircle C' O P ->
OS A B C C' -> Coplanar A B C O ->
CongA A C B A C' B.