a_pow
sum ^= a_pow(bch, i*(1 << j));
syn[j] ^= a_pow(bch, (j+1)*(i+s));
elp->c[j+k] ^= a_pow(bch, tmp+l);
u = a_pow(bch, l0+l2+2*(GF_N(bch)-l1));
e = a_pow(bch, l/2);
d = a_pow(bch, 2*l)^gf_mul(bch, b, f)^d;
syn ^= a_pow(bch, m+j*i);