def undo(cursor, undo_tid, self_tid):
    """Undo a transaction.

        Parameters: "undo_tid", the integer tid of the transaction to undo,
        and "self_tid", the integer tid of the current transaction.

        Returns the states copied forward by the undo operation as a
        list of (oid, old_tid).

        May raise UndoError.
        """
