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

  if(!dtd->ExternalID) return Qnil;

  return NOKOGIRI_STR_NEW2(dtd->ExternalID);
}