async add(filename, document) {
    const PUNCTUATION = ['.', ',', ':', ''];
    const tokenizer = new natural.WordTokenizer();
    const tokens = tokenizer.tokenize(document);
    // filter out punctuation, then add all tokens to a redis set.
    await Promise.all(
      tokens
        .filter(token => PUNCTUATION.indexOf(token) === -1)
        .map(token => this.tokenClient.sAdd(token, filename))
    );
    await this.docsClient.set(filename, document);
  }
Tokenize the given document.
@param {string} filename - key for the storage in redis
@param {string} document - Collection of words to be tokenized
@returns {Promise<void>}