source 1987 Montgomery "Speeding the Pollard and elliptic curve methods of factorization", page 261, fifth display, plus common-subexpression elimination, plus assumption Z1=1
assume Z1 = 1
compute A = X2+Z2
compute B = X2-Z2
compute C = X3+Z3
compute D = X3-Z3
compute DA = D A
compute CB = C B
compute X5 = (DA+CB)^2
compute Z5 = X1(DA-CB)^2