(** Euclid Book III Prop 20: In a circle the angle at the centre is double of the angle at the circumference, when the angles have the same circumference as base. *)

Lemma inscribed_angle : forall O P A B C,
OnCircle A O P -> OnCircle B O P -> OnCircle C O P -> OS A B O C ->
gHalfA A C B A O B.