assume Z2 = 1
compute X1Y2 = X1 Y2
compute Y1X2 = Y1 X2
compute Z1X2 = Z1 X2
compute Z1Y2 = Z1 Y2
compute X3 = Y1X2 Y1-Z1Y2 X1Y2
compute Y3 = X1 X1Y2-Y1X2 Z1X2
compute Z3 = Z1Y2 Z1X2-X1 Y1