source 2002 Brier--Joye "Weierstrass elliptic curves and side-channel attacks", formula (9) accompanied by note "7 multiplications plus 3 multiplications by a constant", plus common-subexpression elimination
assume Z1 = 1
parameter b4
assume b4 = 4*b
compute A = X2 X3
compute B = Z2 Z3
compute C = X2 Z3
compute D = Z2 X3
compute X4 = (A - a B)^2 - b4 B (C + D)
compute Z4 = X1 (C - D)^2
