Section Circle_2D.
Context `{T2D:Tarski_2D}.
(** The center of a circle belongs to the perpendicular bisector of each chord *)

Lemma mid_onc2_perp__col : forall O P A B X Y,
A <> B -> OnCircle A O P -> OnCircle B O P -> Midpoint X A B -> Perp X Y A B -> Col X Y O.