protected function _arrayPermute($items, $perms = []) {
		static $permuted;

		if (empty($perms)) {
			$permuted = [];
		}

		if (empty($items)) {
			$permuted[] = $perms;
			return;
		}
		$numItems = count($items) - 1;

		for ($i = $numItems; $i >= 0; --$i) {
			$newItems = $items;
			$newPerms = $perms;
			list($tmp) = array_splice($newItems, $i, 1);
			array_unshift($newPerms, $tmp);
			$this->_arrayPermute($newItems, $newPerms);
		}
		return $permuted;
	}