source 2002 Izu--Takagi "A fast parallel elliptic curve multiplication resistant against side channel attacks", formulas (8) and (10), plus common-subexpression elimination
parameter b4
assume b4 = 4*b
compute XX = X2^2
compute ZZ = Z2^2
compute aZZ = a ZZ
compute E = (X2 + Z2)^2 - XX - ZZ
compute X4 = (XX - aZZ)^2 - b4 E ZZ
compute Z4 = 2 E(XX + aZZ) + b4 ZZ^2
compute A = X2 X3
compute B = Z2 Z3
compute C = X2 Z3
compute D = X3 Z2
compute X5 = Z1 ((A - a B)^2 - b4 B(C + D))
compute Z5 = X1 (C - D)^2
