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

Lemma l12_9 : forall A1 A2 B1 B2 C1 C2,
Coplanar C1 C2 A1 B1 -> Coplanar C1 C2 A1 B2 ->
Coplanar C1 C2 A2 B1 -> Coplanar C1 C2 A2 B2 ->
Perp A1 A2 C1 C2 -> Perp B1 B2 C1 C2 ->
Par A1 A2 B1 B2.