Section PARALLEL.
(* For each point P and each line D, there exists at most one line through P parallel to D. *)
(* The parallel is unique. *)

Theorem Parallel_unicity : forall A : Point, forall (F : Figure) (D : Line F),
forall (F' F'' : Figure) (D' : Line F') (D'' : Line F''),
ParallelLines F F' D D' ->
F' A ->
ParallelLines F F'' D D'' ->
F'' A ->
Superimposed F' F''.