source 2008 Hisil--Wong--Carter--Dawson, http://eprint.iacr.org/2008/522, Section 3.1
compute A = X1 X2
compute B = Y1 Y2
compute C = T1 d T2
compute D = Z1 Z2
compute E = (X1+Y1)(X2+Y2)-A-B
compute F = D - C
compute G = D + C
compute H = B - a A
compute X3 = E F
compute Y3 = G H
compute T3 = E H
compute Z3 = F G