Section CONGRUENCE.
(* C1 : Given a line segment AB, and given a ray r originated at a point C, there exists a unique point D on the ray r such that AB = CD. *)
(* C2 : If AB = CD and AB = EF, then CD = EF. Every line segment is congruent to itself. *)

Lemma C2a : forall A B C D E F : Point,
Distance A B = Distance C D ->
Distance A B = Distance E F ->
Distance C D = Distance E F.