Ltac lg_instance l A B :=
  assert(tempo_sg:= ex_points_lg l);
  match goal with
    |H: Q_Cong l |-  _ => assert(tempo_H:=H); apply tempo_sg in tempo_H; elim tempo_H; intros A ; let tempoHP := fresh "tempo_HP" in intro tempoHP; clear tempo_H; elim tempoHP; intro B; intro; clear tempoHP
  end;
  clear tempo_sg.

Lemma is_len_cong : forall A B C D l, Len A B l -> Len C D l -> Cong A B C D.