source 2003 Stam "On Montgomery-like representations for elliptic curves over GF(2^k)", Section 3.2, plus a1 = 1, plus b8 = a6, plus common-subexpression elimination
compute A = X2 X3
compute B = Z2 Z3
compute X5 = Z1(A^2 + a6 B^2)
compute Z5 = X1((X2+Z2)(X3+Z3)-A-B)^2