Section Grid.
Variable O E E' : Tpoint.
Variable grid_ok : ~ Col O E E'.
(** We are not faithful to Tarski's def because for uniqueness we do not need the assumption that A and B are on line OE as it is implied by the definition of sum. *)

Lemma grid_not_par : ~Par O E E E' /\ ~Par O E O E' /\ ~Par O E' E E' /\ O <> E /\ O <> E' /\ E <> E'.