proposition: Lemma is_len_cong_is_len : forall A B C D l, Len A B l -> Cong A B C D -> Len C D l.
proof: Proof. intros. unfold Len in *. spliter. split. auto. unfold Q_Cong in H. ex_and H a. ex_and H2 b. assert(HH:= H A B). destruct HH. assert(HH1:= H C D). destruct HH1. apply H3 in H1. apply H4. apply cong_transitivity with A B; trivial. Qed.
imports: GeoCoq.Tarski_dev.Ch13_1