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 X5 = (A - a B)^2 - b4 B (C + D) compute Z5 = X1 (C - D)^2