docs: protected VocabCache<ShallowSequenceElement> buildShallowVocabCache(Counter<Long> counter) {
  
          // TODO: need simplified cache here, that will operate on Long instead of string labels
          VocabCache<ShallowSequenceElement> vocabCache = new AbstractCache<>();
          for (Long id : counter.keySet()) {
              ShallowSequenceElement shallowElement = new ShallowSequenceElement(counter.getCount(id), id);
              vocabCache.addToken(shallowElement);
          }
  
          // building huffman tree
          Huffman huffman = new Huffman(vocabCache.vocabWords());
          huffman.build();
          huffman.applyIndexes(vocabCache);
  
          return vocabCache;
      }