static String checkSystemLiteral(final String systemLiteral) {
    String reason;

    if (systemLiteral == null) return null;
    // This indicates there is no system ID

    if (systemLiteral.indexOf('\'') != -1
        && systemLiteral.indexOf('"') != -1) {
      reason =
        "System literals cannot simultaneously contain both single and double quotes.";
    }
    else {
      reason = checkCharacterData(systemLiteral);
    }

    return reason;
  }
static String checkSystemLiteral(final String systemLiteral)