code: public FP64 extend(char[] chars, int start, int len) {
      int end = start + len;
      for (int i = start; i < end; i++) {
        extend(chars[i]);
      }
      return this;
    }
docs: Extends this fingerprint by the characters
  <code>chars[start]..chars[start+length-1]</code>.
  
  @return
  the resulting fingerprint.