gf_mul
g->c[j] = gf_mul(bch, g->c[j], r)^g->c[j-1];
g->c[0] = gf_mul(bch, g->c[0], r);
d ^= gf_mul(bch, elp->c[j], syn[2*i+2-j]);
c = gf_mul(bch, a2, c2); /* c = a2c2 */
b = gf_mul(bch, a2, b2)^c2; /* b = a2b2 + c2 */
d = a_pow(bch, 2*l)^gf_mul(bch, b, f)^d;
b = gf_mul(bch, a, e)^b;