'Returns all immediate dependencies of a given dependant.
>>> tracker = DependencyTracker()
>>> tracker.add(0, 1)
>>> tracker.add(0, 2)
>>> tracker.add(1, 2)
>>> tracker.get_dependencies(0)
set()
>>> tracker.get_dependencies(1)
{0}
>>> tracker.get_dependencies(2)
{0, 1}
:param dependant:
The dependant to retrieve all dependencies from.
:return:
A set of dependencies.'

def get_dependencies(self, dependant):

    return set((dependency for (dependency, dependants) in self._dependency_dict.items() if (dependant in dependants)))
