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

Lemma perp_inter_perp_in_n
: forall A B C D : Tpoint,
Perp A B C D ->
exists P : Tpoint, Col A B P /\ Col C D P /\ Perp_at P A B C D.