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

Lemma perp2_perp_in : forall A B C D O, Perp2 A B C D O -> ~Col O A B /\ ~Col O C D ->
exists P, exists Q, Col A B P /\ Col C D Q /\ Col O P Q /\ Perp_at P O P A B /\ Perp_at Q O Q C D.