public function getChildren(User $user)
    {
        $returnArray = array();
        foreach ($this->userList as $u) {
            /** @var $u User */
            if ($u->getParent() == $user->getUsername()) {
                $returnArray[] = $u;
                $returnArray = array_merge($returnArray, $this->getChildren($u));
            }
        }
        return $returnArray;
    }