(* 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. *)

Lemma C4a : forall A B C D F : Point,
A <> B ->
A <> C ->
D <> F ->
exists E : Point,
D <> E /\
~Clockwise D F E /\
Angle B A C = Angle  E D F.