source 2008.02.25 Hisil--Wong--Carter--Dawson, plus U=XX, plus V=ZZ, plus W=R/2, plus rescaling, plus denominator elimination, plus standard streamlining, plus Z2=1 assume Z2 = 1 parameter k assume k = a-1 compute A = 2 XX1 XX2 compute B = 2 ZZ1 compute C = R1 R2 compute D = Y1 Y2 compute X3 = (R1+Y1)(R2+Y2)-C-D compute Z3 = B-A compute XX3 = X3^2 compute ZZ3 = Z3^2 compute F = A + B + C compute G = 2((XX1+ZZ1)(XX2+1)+D)+k C compute H = XX3+ZZ3 compute Y3 = F G - H compute R3 = (X3+Z3)^2-H