Definition gHalfA A' O' B' A O B := exists P, HalfA P A O B /\ CongA A' O' B' A O P.

Lemma ghalfa_preserves_lta : forall A B C X Y Z A' B' C' X' Y' Z',
gHalfA A' B' C' A B C -> gHalfA X' Y' Z' X Y Z -> LtA A B C X Y Z -> LtA A' B' C' X' Y' Z'.