(** Given two angles a and b, a/2 = b/2 -> a = b *)

Lemma ghalfa_preserves_conga_1 : forall A B C A' B' C' X Y Z X' Y' Z',
gHalfA X Y Z A B C -> gHalfA X' Y' Z' A' B' C' -> CongA X Y Z X' Y' Z' ->
CongA A B C A' B' C'.