docs: def get_objective_bank_nodes(self,
                                   objective_bank_id=None,
                                   ancestor_levels=None,
                                   descendant_levels=None,
                                   include_siblings=None):
          """Gets a portion of the hierarchy for the given objective bank.
  
          arg:    includeSiblings (boolean): true to include the siblings
                  of the given node, false to omit the siblings
          return: (osid.learning.ObjectiveBankNode) - an objective bank
                  node
          raise:  NotFound - objectiveBankId not found
          raise:  NullArgument - objectiveBankId is null
          raise:  OperationFailed - unable to complete request
          raise:  PermissionDenied - authorization failure
          compliance: mandatory - This method must be implemented.
  
          """
          if descendant_levels:
              url_path = self._urls.nodes(alias=objective_bank_id, depth=descendant_levels)
          else:
              url_path = self._urls.nodes(alias=objective_bank_id)
          return self._get_request(url_path)