(* Parallelogram axiom *)

(* Specifications: *)
Axiom parallel_side_eq_parallel : forall P Q C D,
parallel P Q C D -> P**Q=C**D -> C<>D -> parallel D Q P C.
(** We assume that the field is not of characteristic two *)
Axiom chara_not_2 : 2 <> 0.
Hint Unfold S4 parallel Col: Geom.