public function set(UserInterface $fromUser, UserInterface $toUser)
    {
        $event = new ApplySuccessorEvent($fromUser, $toUser);
        $this->dispatcher->dispatch(UserEvents::APPLY_SUCCESSOR, $event);

        $message = UsersMessage::create(
            sprintf('Set "%s" as successor for "%s".', $toUser->getUsername(), $fromUser->getUsername())
        );
        $this->messageService->post($message);
    }