(** If the angle ACB is inscribed in a circle of center O and C, O lie on the same side of AB, then this angle is acute. *)

Lemma inscribed_angle_aux1 : forall O P A B C,
OnCircle A O P -> OnCircle B O P -> OnCircle C O P ->
OS A B O C -> OS O C A B ->
gHalfA A C B A O B.