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

Lemma perp2_preserves_bet23 : forall O A B A' B', Bet O A B -> Col O A' B' -> ~Col O A A' ->
Perp2 A A' B B' O -> Bet O A' B'.