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. *)
(* C3 : Given three points A, B, C on a line satisfying A - B - C, and three further points D, E, F on a line satisfying D - E - F, if AB = DE and BC = EF, then AC = DF. *)
(* C4 : Given an angle ^BAC and given a ray DF, there exists a unique ray DE on a given side of the line DF, such that ^BAC = ^EDF. *)
(* C5 : For any three angles ^A, ^B, ^C, if ^A = ^Bâ and ^A = ^C, then ^B = ^C. Every angle is congruent to itself. *)
(* C6 (SAS): Given triangles ABC and DEF, suppose that AB = DE and AC = DF and ^BAC = ^EDF, then the two triangles are congruent, namely, BC = EF, ^ABC = ^DEF and ^ACB = ^DFE. *)

Lemma C6a  : forall A B C D E F : Point,
A <> B ->
A <> C ->
Distance A B = Distance D E ->
Distance A C = Distance D F ->
Angle B A C = Angle E D F ->
Distance B C = Distance E F.