source 2007(?) Kim--Kim "A New Method for Speeding Up Arithmetic on Elliptic Curves over Binary Fields"
compute A = X1^2
compute B = Y1^2
compute Z3 = ZZ1 A
compute ZZ3 = Z3^2
compute X3 = A^2 + a6 ZZ1^2
compute Y3 = B (B + X3 + Z3) + a6 ZZ3 + ZZ3