End T6_1.
Hint Resolve col_transitivity_1 col_transitivity_2 out_col : col.
Section T6_2.
Context `{TnEQD:Tarski_neutral_dimensionless_with_decidable_point_equality}.

(* This is l6_25 of Tarski *)
Lemma not_col_exists : forall A B,
A<>B -> exists C, ~ Col A B C.