source 2007 Feng--Wu, second JQN2 doubling formula, plus epsilon=1, plus delta=-a
appliesto 2xxyzzr
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 = T2-2 a T1-2 X1^4
compute XX3 = X3^2
compute ZZ3 = Z3^2
compute T3 = XX3+ZZ3
compute R3 = (X3+Z3)^2-T3