a_log
r = a_log(bch, y);
tmp = a_log(bch, d)+n-a_log(bch, pd);
l = a_log(bch, pelp->c[j]);
j = a_log(bch, b);
k = a_log(bch, a);
l = a_log(bch, f);
int i, d = a->deg, l = GF_N(bch)-a_log(bch, a->c[a->deg]);
rep[i] = a->c[i] ? mod_s(bch, a_log(bch, a->c[i])+l) : -1;
la = a_log(bch, c[j]);