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.