code: public void addKeyword(String keyword) {
          keywords.put(keywordsCaseSensitive ? keyword.intern() : keyword.toLowerCase().intern(), keyword);
      }
docs: Adds a keyword which is now being recognized by the tokenizer
  <p>
  Detection will be case insensitive. Only ID tokens (identifiers) are checked against the given keywords,
  therefore a keyword must be a valid identifier.
  
  @param keyword the keyword to be added to the list of known keywords.