source 2007 Hisil--Carter--Dawson compute B = XX1-ZZ1 compute T1 = XX1+ZZ1 compute C = Y1 T1 compute X3 = C-Y1(T1+R1) compute Z3 = T1 B compute XX3 = X3^2 compute ZZ3 = Z3^2 compute T3 = XX3+ZZ3 compute R3 = (X3+Z3)^2-T3 compute Y3 = 2 C^2-T3