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.