code: def get_objective_bank_hierarchy(self, alias):
          
          url_path = self._urls.hierarchy(re.sub(r'[ ]', '', alias.lower()))
          return self._get_request(url_path)
docs: Gets the hierarchy associated with this session.
  
          return: (osid.hierarchy.Hierarchy) - the hierarchy associated
                  with this session
          raise:  OperationFailed - unable to complete request
          raise:  PermissionDenied - authorization failure
          *compliance: mandatory -- This method must be implemented.*