<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def getCallSet(self, id_):
 """ Returns a CallSet with the specified id, or raises a CallSetNotFoundException if it does not exist. """

        if id_ not in self._callSetIdMap:
            raise exceptions.CallSetNotFoundException(id_)
        return self._callSetIdMap[id_]