* <ul>
* <li>Tasks can be safely added and removed in the middle of a run.</li>
* <li>No task will be returned more than once during a run.</li>
* <li>No task will be returned when it is not, at that moment, in the list of tasks.</li>
* <li>At the moment when next() returns null, all tasks on the list have been returned during the run.</li>
* <ul>
* <li>Tasks can be safely added and removed in the middle of a run.</li>
* <li>No task will be returned more than once during a run.</li>
* <li>No task will be returned when it is not, at that moment, in the list of tasks.</li>
* <li>At the moment when next() returns null, all tasks on the list have been returned during the run.</li>