Section T8_3.
Context `{TnEQD:Tarski_neutral_dimensionless_with_decidable_point_equality}.

Lemma l8_13_2 : forall A B C D X,
A <> B -> C <> D -> Col X A B -> Col X C D ->
(exists U, exists V :Tpoint, Col U A B /\ Col V C D /\ U<>X /\ V<>X /\ Per U X V) ->
Perp_at X A B C D.