/*
 * call-seq:
 *   system_id
 *
 * Get the System ID for this DTD
 */
static VALUE system_id(VALUE self)
{
  xmlDtdPtr dtd;
  Data_Get_Struct(self, xmlDtd, dtd);

  if(!dtd->SystemID) return Qnil;

  return NOKOGIRI_STR_NEW2(dtd->SystemID);
}