protected function flattenPriorities()
{
if (is_array($this->_fd)) {
return $this->_fd;
}
$this->sortPriorities();
$this->_fd = array();
foreach ($this->_d as $priority => $itemsatpriority) {
$this->_fd = array_merge($this->_fd, $itemsatpriority);
}
return $this->_fd;
}