/**
 * return a reference to a registered object
 */
function &get_registered_object($name) {
   if (!isset($this->_reg_objects[$name]))
   $this->_trigger_fatal_error("'$name' is not a registered object");
   if (!is_object($this->_reg_objects[$name][0]))
   $this->_trigger_fatal_error("registered '$name' is not an object");
   return $this->_reg_objects[$name][0];
}