Axiom
constructed_orthogonal :
forall (l : Line) (a : Point), {l' : Line | Ort l' l /\ Incident a l'}.
Definition ort : Line -> Point -> Line.

Theorem Uniqueness_of_orthogonality :
forall (l m : Line) (a : Point), Incident a l /\ Ort l m -> EqLn l (ort m a).