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 sum_uniquenessB : forall A X Y C, Sum O E E' A X C -> Sum O E E' A Y C -> X = Y.