'Lookup the label ids that an atomic_group is associated with.
:param atomic_group_id - The id of the AtomicGroup to look up.
:return: A generator yielding Label ids for this atomic group.'

def _get_atomic_group_labels(self, atomic_group_id):

    return (a_id for (a_id, label) in self._labels.iteritems() if ((label.atomic_group_id == atomic_group_id) and (not label.invalid)))
