End HalfAngle.
Section HalfAngle_2D.
Context `{T2D:Tarski_2D}.

(* Specifications: *)
Lemma halfa_perp__os : forall P A O B T, HalfA P A O B -> Perp O P T O -> OS O T P A.
Lemma halfa_perp__os2 : forall P A O B T, HalfA P A O B -> Perp O P T O ->
OS O T P A /\ OS O T P B.