End Grid.

(* (x + y) - (a + b) = (x - a) + (y - b) *)
Lemma sum_diff2_diff_sum2_a : forall O E E' A B C X Y Z dXA dYB dZC,
Sum O E E' A B C -> Sum O E E' X Y Z -> Diff O E E' X A dXA ->
Diff O E E' Y B dYB -> Sum O E E' dXA dYB dZC ->
Diff O E E' Z C dZC.