Section T12_2D.
Context `{T2D:Tarski_2D}.

(* Specifications: *)
Lemma col_perp2__col : forall X1 X2 Y1 Y2 A B,
Perp X1 X2 A B -> Perp Y1 Y2 A B -> Col X1 Y1 Y2 -> Col X2 Y1 Y2.
Lemma l12_9_2D : forall A1 A2 B1 B2 C1 C2,
Perp A1 A2 C1 C2 -> Perp B1 B2 C1 C2 -> Par A1 A2 B1 B2.
Lemma conga_inangle_per2__inangle : forall A B C P T,
Per A B C -> InAngle T A B C -> CongA P B A P B C -> Per B P T ->
InAngle P A B C.