int cmp_item_row::compare(cmp_item *c)
{
  cmp_item_row *l_cmp= (cmp_item_row *) c;
  for (uint i=0; i < n; i++)
  {
    int res;
    if ((res= comparators[i]->compare(l_cmp->comparators[i])))
      return res;
  }
  return 0;
}