private void appendExternalId(final String publicID, final String systemID) {
    if (publicID != null) {
      internalSubset.append(" PUBLIC \"").append(publicID).append('\"');
    }
    if (systemID != null) {
      if (publicID == null) {
        internalSubset.append(" SYSTEM ");
      }
      else {
        internalSubset.append(' ');
      }
      internalSubset.append('\"').append(systemID).append('\"');
    }
  }
private void appendExternalId(final String publicID, final String systemID)