<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def add_ring(self, ring):
 """Adds a ring to _rings if not already existing"""

        if ring not in self._rings and isinstance(ring, RingDing0):
            self._rings.append(ring)