code: private Mention getMention(int tokenIndex)
      {
          Mention mention = null;
          for (Mention mention2 : mentions)
          {
              if (mention2.contains(tokenIndex))
                  if (mention == null)
                      mention = mention2;
                  else
                      throw new IllegalArgumentException();
          }
          return mention;
      }
docs: Assumes that each token is tagged either 0 or 1 times