source 2007 Hisil--Carter--Dawson assume Z1 = 1 compute A = XX1^2 compute B = Y1^2 compute X3 = XX1+B-(X1+Y1)^2 compute Z3 = A-1 compute XX3 = X3^2 compute ZZ3 = Z3^2 compute T3 = XX3+ZZ3 compute R3 = (X3+Z3)^2-T3 compute Y3 = 2 B(A+2 XX1+1)-T3