function enumerateProject(Project $project)
{
    print "{$project->getTitle()}({$project->getSummary()}): \n";
    foreach ($project->getToDos() as $eachToDo) {
        if ($eachToDo instanceof Project) {
            print "sub:";
            enumerateProject($eachToDo);
        } else {
            print "{$eachToDo->getTitle()}\n";
        }
    }
}