End Grid.

(* (b - a) + (c - b) = (c - a) *)
Lemma sum_diff_diff_a : forall O E E' A B C dBA dCB dCA,
Diff O E E' B A dBA -> Diff O E E' C B dCB -> Diff O E E' C A dCA ->
Sum O E E' dCB dBA dCA.