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

Lemma ghalfa_preserves_conga_2 : 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 A B C A' B' C' ->
CongA X Y Z X' Y' Z'.