source 2007 Feng--Wu, second JQN2 doubling formula, plus epsilon=1, plus delta=-a appliesto 2xxyzz parameter a2 assume a2 = 2 a compute T1 = (X1 Z1)^2 compute T2 = Y1^2 compute S1 = (T2+2 c T1)^2 compute U = (X1 Z1+Y1)^2-T1-T2 compute Q = U^2 compute X3 = U compute Y3 = S1-c Q compute Z3 = T2-a2 T1-2 X1^4 compute XX3 = X3^2 compute ZZ3 = Z3^2