End T12_2.
Section T12_2'.
Context `{TnEQD:Tarski_neutral_dimensionless_with_decidable_point_equality}.

Lemma col_cop2_perp2__col : forall X1 X2 Y1 Y2 A B,
Perp X1 X2 A B -> Perp Y1 Y2 A B -> Col X1 Y1 Y2 ->
Coplanar A B X2 Y1 -> Coplanar A B X2 Y2 -> Col X2 Y1 Y2.