source 2008 Hisil--Wong--Carter--Dawson, http://eprint.iacr.org/2008/522, Section 3.2
appliesto extended-1
assume Z2 = 1
compute A = (Y1-X1)(Y2+X2)
compute B = (Y1+X1)(Y2-X2)
compute C = Z1 2 T2
compute D = 2 T1
compute E = D + C
compute F = B - A
compute G = B + A
compute H = D - C
compute X3 = E F
compute Y3 = G H
compute T3 = E H
compute Z3 = F G