proposition: Lemma l4_3 : forall A B C A' B' C', Bet A B C -> Bet A' B' C' -> Cong A C A' C' -> Cong B C B' C' -> Cong A B A' B'.
proof: Proof. intros. apply cong_commutativity. apply (l4_2 A B C A A' B' C' A'). unfold IFSC. repeat split;Cong. Qed.
imports: GeoCoq.Tarski_dev.Ch03_bet GeoCoq.Tarski_dev.Tactics.CongR