<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def current(self):
 """Return the topmost transaction. .. note:: If the topmost element on the stack is not a transaction, returns None. :rtype: :class:`google.cloud.datastore.transaction.Transaction` or None :returns: The current transaction (if any are active). """

        top = super(Transaction, self).current()
        if isinstance(top, Transaction):
            return top