docs: def get_relationship_family_assignment_session(self, proxy=None):
          """Gets the ``OsidSession`` associated with assigning relationships to families.
  
          arg:    proxy (osid.proxy.Proxy): a proxy
          return: (osid.relationship.RelationshipFamilyAssignmentSession)
                  - a ``RelationshipFamilyAssignmentSession``
          raise:  NullArgument - ``proxy`` is ``null``
          raise:  OperationFailed - unable to complete request
          raise:  Unimplemented -
                  ``supports_relationship_family_assignment()`` is
                  ``false``
          *compliance: optional -- This method must be implemented if ``supports_relationship_family_assignment()``
              is ``true``.*
  
          """
          if not self.supports_relationship_family_assignment():
              raise Unimplemented()
          try:
              from . import sessions
          except ImportError:
              raise OperationFailed()
          proxy = self._convert_proxy(proxy)
          try:
              session = sessions.RelationshipFamilyAssignmentSession(proxy=proxy, runtime=self._runtime)
          except AttributeError:
              raise OperationFailed()
          return session