code: def get_vault_hierarchy_session(self):
          
          if not self.supports_vault_hierarchy():
              raise errors.Unimplemented()
          # pylint: disable=no-member
          return sessions.VaultHierarchySession(runtime=self._runtime)
docs: Gets the session traversing vault hierarchies.
  
          return: (osid.authorization.VaultHierarchySession) - a
                  ``VaultHierarchySession``
          raise:  OperationFailed - unable to complete request
          raise:  Unimplemented - ``supports_vault_hierarchy() is false``
          *compliance: optional -- This method must be implemented if
          ``supports_vault_hierarchy()`` is true.*