Section L13_1.
Context `{TnEQD:Tarski_neutral_dimensionless_with_decidable_point_equality}.
(** Pappus Desargues *)

Lemma per2_preserves_diff : forall O A B A' B', O <> A' -> O <> B' -> Col O A' B' -> Per O A' A -> Per O B' B -> A' <> B' -> A <> B.