Section T10.
Context `{TnEQD:Tarski_neutral_dimensionless_with_decidable_point_equality}.
Goal forall A B C D, ~ Coplanar D C B A -> ~ Coplanar A B C D.

Lemma ex_sym : forall A B X, exists Y, (Perp A B X Y \/ X = Y) /\
(exists M, Col A B M /\ Midpoint M X Y).