source 2008.02.25 Hisil--Wong--Carter--Dawson, page 6 assume Z2 = 1 compute SC1 = S1 C1 compute DZ1 = D1 Z1 compute SC2 = S2 C2 compute E = S1 D2 compute G = D1 S2 compute H = Z1 C2 compute J = SC1 D2 compute K = DZ1 SC2 compute S3 = (H+C1)(E+G)-J-K compute C3 = (H+E)(C1-G)-J+K compute D3 = (DZ1-a SC1)(SC2+D2)+a J-K compute Z3 = (H+G)^2-2 K