(* C2 : If AB = CD and AB = EF, then CD = EF. Every line segment is congruent to itself. *)

(* Specifications: *)
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.
Lemma C2b : forall A B : Point,
Distance A B = Distance A B.