/*
 * call-seq:
 *  external_id
 *
 * Get the external identifier for PUBLIC
 */
static VALUE external_id(VALUE self)
{
  xmlEntityPtr node;
  Data_Get_Struct(self, xmlEntity, node);

  if(!node->ExternalID) return Qnil;

  return NOKOGIRI_STR_NEW2(node->ExternalID);
}