assume Z1 = 1 assume Z2 = 1 compute X1Y2 = X1 Y2 compute Y1X2 = Y1 X2 compute X3 = Y1X2 Y1-Y2 X1Y2 compute Y3 = X1 X1Y2-Y1X2 X2 compute Z3 = Y2 X2-X1 Y1