'Returns a set of all registered dependants.
>>> tracker = DependencyTracker()
>>> tracker.add(0, 1)
>>> tracker.add(0, 2)
>>> tracker.add(1, 3)
>>> tracker.dependants
{1, 2, 3}'

@property
def dependants(self):

    return set(chain.from_iterable(self._dependency_dict.values()))
