Proof with Tarski.
intros.
DecompExAnd H X.
unfold le.
assert (exists B' : Point, Bet C B' D /\ Cong_3 A B X C B' D)...
DecompExAnd H Y.
exists Y.
intuition.
unfold Cong_3 in H4;intuition;sTarski.
Qed.

Lemma l5_6 : forall A B C D A' B' C' D',
le A B C D /\ Cong A B A' B' /\ Cong C D C' D' -> le A' B' C' D'.