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

Lemma perp_in_rewrite : forall A B C D P, Perp_at P A B C D ->
Perp_at P A P P C \/
Perp_at P A P P D \/
Perp_at P B P P C \/
Perp_at P B P P D.