docs: def get_composition_repository_assignment_session(self, proxy):
          """Gets the session for assigning composition to repository
          mappings.
  
          arg     proxy (osid.proxy.Proxy): a proxy
          return: (osid.repository.CompositionRepositoryAssignmentSession)
                  - a CompositionRepositoryAssignmentSession
          raise:  OperationFailed - unable to complete request
          raise:  Unimplemented -
                  supports_composition_repository_assignment() is false
          compliance: optional - This method must be implemented if
                      supports_composition_repository_assignment() is
                      true.
  
          """
          if not self.supports_composition_repository_assignment():
              raise Unimplemented()
          try:
              from . import sessions
          except ImportError:
              raise  # OperationFailed()
          proxy = self._convert_proxy(proxy)
          try:
              session = sessions.CompositionRepositoryAssignmentSession(proxy, runtime=self._runtime)
          except AttributeError:
              raise  # OperationFailed()
          return session