docs: def get_asset_composition_session(self):
          """Gets the session for retrieving asset compositions.
  
          return: (osid.repository.AssetCompositionSession) - an
                  AssetCompositionSession
          raise:  OperationFailed - unable to complete request
          raise:  Unimplemented - supports_asset_composition() is false
          compliance: optional - This method must be implemented if
                      supports_asset_composition() is true.
  
          """
          if not self.supports_asset_composition():
              raise Unimplemented()
          try:
              from . import sessions
          except ImportError:
              raise  # OperationFailed()
          try:
              session = sessions.AssetCompositionSession(proxy=self._proxy,
                                                         runtime=self._runtime)
          except AttributeError:
              raise  # OperationFailed()
          return session