source 1987 Montgomery "Speeding the Pollard and elliptic curve methods of factorization", page 261, fifth and sixth displays
parameter a24
assume a24 = (a+2)/4
compute X5 = Z1((X3-Z3)(X2+Z2)+(X3+Z3)(X2-Z2))^2
compute Z5 = X1((X3-Z3)(X2+Z2)-(X3+Z3)(X2-Z2))^2
compute X4 = (X2+Z2)^2 (X2-Z2)^2
compute Z4 = (4 X2 Z2)((X2-Z2)^2 + a24(4 X2 Z2))
