source 1987 Montgomery "Speeding the Pollard and elliptic curve methods of factorization", page 261, sixth display
parameter a24
assume a24 = (a+2)/4
compute X3 = (X1+Z1)^2 (X1-Z1)^2
compute Z3 = (4 X1 Z1)((X1-Z1)^2 + a24(4 X1 Z1))
