gf_sqr
y = gf_sqr(bch, x)^x;
syn[2*j+1] = gf_sqr(bch, syn[j]);
if ((gf_sqr(bch, r)^r) == u) {
a = gf_sqr(bch, a2)^b2; /* a = a2^2 + b2 */
z->c[2*j] = gf_sqr(bch, z->c[j]);