code: def add_root_objective_bank(self, alias=None, objective_bank_id=None):
          
          url_path = self._urls.roots(alias=alias)
          current_root_ids = self._get_request(url_path)['ids']
          current_root_ids.append(str(objective_bank_id))
          new_root_ids = {
              'ids': current_root_ids
          }
          return self._put_request(url_path, new_root_ids)
docs: Adds a root objective bank.
  
          arg:    objective_bank_id (osid.id.Id): the ``Id`` of an
                  objective bank
          raise:  AlreadyExists - ``objective_bank_id`` is already in
                  hierarchy
          raise:  NotFound - ``objective_bank_id`` not found
          raise:  NullArgument - ``objective_bank_id`` is ``null``
          raise:  OperationFailed - unable to complete request
          raise:  PermissionDenied - authorization failure
          *compliance: mandatory -- This method must be implemented.*