code: private String digits() {
      StringBuilder sb = new StringBuilder();
      do {
        sb.append(consumeNextCharacter(CharType.DIGIT));
      } while (chars.positiveLookahead(CharType.DIGIT));
      return sb.toString();
    }
docs: Parses the {@literal <digits>} non-terminal.
  
  <pre>
  {@literal
  <digits> ::= <digit>
  | <digit> <digits>
  }
  </pre>
  
  @return a string representing the digits