End Grid.

Lemma sum_diff_diff : forall O E E' AX BX CX AXMBX AXMCX BXMCX,
Diff O E E' AX BX AXMBX -> Diff O E E' AX CX AXMCX ->
Diff O E E' BX CX BXMCX ->
Sum O E E' AXMBX BXMCX AXMCX.