source 2008.02.25 Hisil--Wong--Carter--Dawson, page 6
assume Z2 = 1
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
compute SC3 = S3 C3
compute DZ3 = D3 Z3