Exactly - and being of a mathematical persuasion, I very nearly posted exactly that [I was actually going to type "X = A + B + C + X", but for some reason decided not to include it]. Great mindsSimply put -
A = B + C + D + A
is not possible.
Of course, if your B, C & D (or my A, B & C) were all zero/'nothing', then the equation would actually work
Kind Regards, John