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

Lemma l13_1 : forall A B C P Q R,
~ Col A B C -> Midpoint P B C -> Midpoint Q A C -> Midpoint R A B ->
exists X, exists Y, Perp_at R X Y A B /\ Perp X Y P Q.