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.
Section Length_2.
Context `{TnEQD:Tarski_neutral_dimensionless_with_decidable_point_equality}.

Lemma lg_null_dec : forall l, Q_Cong l -> Q_Cong_Null l \/ ~ Q_Cong_Null l.