docs: def get_comment_book_session(self):
          """Gets the session for retrieving comment to book mappings.
  
          return: (osid.commenting.CommentBookSession) - a
                  ``CommentBookSession``
          raise:  OperationFailed - unable to complete request
          raise:  Unimplemented - ``supports_comment_book()`` is ``false``
          *compliance: optional -- This method must be implemented if
          ``supports_comment_book()`` is ``true``.*
  
          """
          if not self.supports_comment_book():
              raise errors.Unimplemented()
          # pylint: disable=no-member
          return sessions.CommentBookSession(runtime=self._runtime)