Section Circle_2.
Context `{TnEQD:Tarski_neutral_dimensionless_with_decidable_point_equality}.
(** If a point is strictly inside a segment of a disk, it is strictly inside the circle. *)
(** If A and B are two points inside the circle, then all points on the segment AB are also in the circle, i.e. a circle is a convex figure. *)
(** Given two points U and V on a circle, the points of the line UV which are inside the circle are between U and V. *)
(** Given two points U and V on a circle, all points of line UV which are outside the segment UV are outside the circle. *)
(** Given two points U and V inside a circle, all points of line UV which are outside the circle are outside the segment UV. *)
(** If the center of a circle belongs to a chord, then it is the midpoint of the chord. *)
(** Given a point U on a circle and a point P inside the circle, there is a point V such as UV is a chord of the circle going through P. *)
(** Given a circle, there is a point strictly outside the circle. *)
(** Given a circle of center O and a ray OX, there is a point on the ray which is also strictly outside the circle. *)
(** Given a circle there is a point which is strictly inside. *)
(** Given a circle of center O and a ray OX, there is a point on the ray which is also strictly inside the circle. *)
(** Given a circle of center O and a ray OX, there is a point on the ray which is also on the circle. *)
(** Given a circle of center O and a line OX, O is between two points of the line which are also on the circle. *)
(** The symmetric of a point on a circle relative to the center is also on the circle. *)
(** The middle of a chord together with the center of the circle and one end of the chord form a right angle *)
(** Euclid Book III Prop 3 (two lemmas). If a straight line passing through the center of a circle bisects a straight line not passing through the center, then it also cuts it at right angles; and if it cuts it at right angles, then it also bisects it. *)
(** The line from the center of a circle to the midpoint of a chord is perpendicular to the chord. *)
(** If a line passing through the center of a circle is perpendicular to a chord, then they intersect at the middle of the chord *)

Lemma col_onc2_perp__mid : forall O P A B X,
O<>X -> A<>B -> Col A B X -> OnCircle A O P -> OnCircle B O P -> Perp O X A B -> Midpoint X A B.