code: @Override
      public int indexOf(String label) {
          T token = tokenFor(label);
          if (token != null) {
              return token.getIndex();
          } else
              return -2;
      }
docs: Returns Huffman index for specified label
  
  @param label the label to get index for
  @return >=0 if label exists, -1 if Huffman tree wasn't built yet, -2 if specified label wasn't found