<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def getReadGroup(self, id_):
 """ Returns the ReadGroup with the specified id if it exists in this ReadGroupSet, or raises a ReadGroupNotFoundException otherwise. """

        if id_ not in self._readGroupIdMap:
            raise exceptions.ReadGroupNotFoundException(id_)
        return self._readGroupIdMap[id_]