source 2008.02.25 Hisil--Wong--Carter--Dawson, page 8
assume Z2 = 1
compute A = X1
compute B = Y1
compute C = Z1 X2
compute D = Z1 Y2
compute E = A B
compute F = C D
compute G = E+F
compute H = E-F
compute X3 = ((A+D)(B+C)-G)H
compute Y3 = ((A-C)(B+D)-H)G
compute Z3 = c G H