<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def cardinality(self):
 """ This is an over-approximation of the cardinality of this DSIS. :return: """

        cardinality = 0
        for si in self._si_set:
            cardinality += si.cardinality
        return cardinality