private void loadFrequentCfMap(int n) {
   if (frequentTermsCfs != null) return;
   frequentTermsCfs = new HMapKL<String>();
   if (cfs.length < n) n = cfs.length;
   for (int id = 1; id <= n; id++) {
     frequentTermsCfs.put(prefixSet.getTerm(idToTerm[id - 1]), cfs[idToTerm[id - 1]]);
   }
 }