source 1987 Montgomery "Speeding the Pollard and elliptic curve methods of factorization", page 261, sixth display, plus common-subexpression elimination
parameter a24
assume 4 a24 = a+2
compute A = X1+Z1
compute AA = A^2
compute B = X1-Z1
compute BB = B^2
compute C = AA-BB
compute X3 = AA BB
compute Z3 = C(BB + a24 C)