End Grid.

Lemma sum_diff2_diff_sum2_b : 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 -> Diff O E E' Z C dZC ->
Sum O E E' dXA dYB dZC .