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

Lemma perp2_preserves_bet13 : forall O B C B' C', Bet B O C -> Col O B' C' -> ~Col O B B' ->
Perp2 B C' C B' O -> Bet B' O C'.