Proof with Tarski.
intros.
assert (exists D',  Bet C' A' D' /\ A' <> D')...
DecompExAnd H1 D'.
assert (exists B',  Bet D' A' B' /\ Cong A' B' A B)...
DecompExAnd H1 B'.
assert (exists C'',  Bet D' B' C'' /\ Cong B' C'' B C)...
DecompExAnd H1 C''.
assert (Bet A' B' C'');Between.
assert (Bet D' A' C'');Between.
assert (Cong A C A' C'').
eapply l2_11.
apply H.
apply H1.
sTarski.
sTarski.
assert (C''=C').
assert (D' <> A').
auto.
eapply construction_unicity.
apply H10.
eauto.
apply cong_symmetry.
apply H9.
sTarski.
sTarski.
exists B'.
unfold Cong_3.
intuition.
subst C'...
subst C''.
sTarski.
Qed.
Hint Resolve l4_5 : Tarski.

(* Specifications: *)
Lemma l4_6 : forall A B C A' B' C',
Bet A B C -> Cong_3 A B C A' B' C' ->  Bet A' B' C'.