@abc.abstractmethod<EOL><INDENT>def get_families(self):<DEDENT>
return<EOL>
Gets all families.

        In plenary mode, the returned list contains all known families
        or an error results. Otherwise, the returned list may contain
        only those families that are accessible through this session.

        :return: a list of families
        :rtype: ``osid.relationship.FamilyList``
        :raise: ``OperationFailed`` -- unable to complete request
        :raise: ``PermissionDenied`` -- authorization failure

        *compliance: mandatory -- This method must be implemented.*