/**
* Returns the unique key of the current leaf.
*
* @return null|int|string Returns the unique key of the current leaf on
* success, null otherwise
*/
public function key()
{
if (is_null($this->current)) {
$this->current = current($this->branch);
}
if ($this->current instanceof RecursiveLeafInterface) {
return $this->current->key();
}
}
