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 opp_uniqueness :
forall A MA1 MA2,
Opp O E E' A MA1 ->
Opp O E E' A MA2 ->
MA1 = MA2.