protected function injectInCrown(RecursiveLeafInterface $leaf, $uniqueKey = null)
    {
        if (is_null($uniqueKey)) {
            $this->crown[] = null;

            $uniqueKey = key(array_slice($this->crown, -1, 1, true));
        }
        $leaf->setUniqueKey($uniqueKey);
        $this->crown[$uniqueKey] = $leaf;
    }