source 2008.02.25 Hisil--Wong--Carter--Dawson, page 10, top assume Z2 = 1 compute X3 = YY1 XZ2 - XZ1 YY2 compute Y3 = XX1 YZ2 - YZ1 XX2 compute Z3 = ZZ1 XY2 - XY1 compute XX3 = X3^2 compute YY3 = Y3^2 compute ZZ3 = Z3^2 compute XY3 = (X3+Y3)^2-XX3-YY3 compute XZ3 = (X3+Z3)^2-XX3-ZZ3 compute YZ3 = (Y3+Z3)^2-YY3-ZZ3