(* Gabriel Braun April 2013 *)
Require Export GeoCoq.Tarski_dev.Ch13_1.
Section Length_1.
Context `{TnEQD:Tarski_neutral_dimensionless_with_decidable_point_equality}.
(** Pappus Desargues *)
(******************** length *********************)

Lemma lg_sym : forall l A B, Q_Cong l -> l A B -> l B A.