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:  NullArgument - ``proxy`` is ``null``
          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 errors.Unimplemented()
          # pylint: disable=no-member
          return sessions.CompositionRepositoryAssignmentSession(proxy=proxy, runtime=self._runtime)