Hint Resolve l8_5 : perp.
Ltac let_symmetric C P A :=
let id1:=fresh in (assert (id1:(exists A', Midpoint P A A'));
[apply symmetric_point_construction|ex_and id1 C]).
Ltac symmetric B' A B :=
assert(sp:= symmetric_point_construction B A); ex_and sp B'.
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.