source 2008.02.25 Hisil--Wong--Carter--Dawson, page 10, top compute XX1 = X1^2 compute YY1 = Y1^2 compute ZZ1 = Z1^2 compute XY1 = (X1+Y1)^2-XX1-YY1 compute XZ1 = (X1+Z1)^2-XX1-ZZ1 compute YZ1 = (Y1+Z1)^2-YY1-ZZ1 compute XX2 = X2^2 compute YY2 = Y2^2 compute ZZ2 = Z2^2 compute XY2 = (X2+Y2)^2-XX2-YY2 compute XZ2 = (X2+Z2)^2-XX2-ZZ2 compute YZ2 = (Y2+Z2)^2-YY2-ZZ2 compute X3 = YY1 XZ2 - XZ1 YY2 compute Y3 = XX1 YZ2 - YZ1 XX2 compute Z3 = ZZ1 XY2 - XY1 ZZ2