docs: public void truncateVocabulary(int threshold) {
          logger.debug("Truncating vocabulary to minWordFrequency: [" + threshold + "]");
          Set<String> keyset = vocabulary.keySet();
          for (String word : keyset) {
              VocabularyWord vw = vocabulary.get(word);
  
              // please note: we're not applying threshold to SPECIAL words
              if (!vw.isSpecial() && vw.getCount() < threshold) {
                  vocabulary.remove(word);
                  if (vw.getHuffmanNode() != null)
                      idxMap.remove(vw.getHuffmanNode().getIdx());
              }
          }
      }