docs: def get_bank_hierarchy_session(self):
          """Gets the session traversing bank hierarchies.
  
          return: (osid.assessment.BankHierarchySession) - a
                  ``BankHierarchySession``
          raise:  OperationFailed - unable to complete request
          raise:  Unimplemented - ``supports_bank_hierarchy() is false``
          *compliance: optional -- This method must be implemented if
          ``supports_bank_hierarchy()`` is true.*
  
          """
          if not self.supports_bank_hierarchy():
              raise errors.Unimplemented()
          # pylint: disable=no-member
          return sessions.BankHierarchySession(runtime=self._runtime)