source 2007 Bernstein--Lange
assume Z1 = 1
assume Z2 = 1
compute C = X1 X2
compute D = Y1 Y2
compute E = d C D
compute X3 = (1-E)((X1+Y1)(X2+Y2)-C-D)
compute Y3 = (1+E)(D-C)
compute Z3 = c(1-E^2)