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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:

def pool(self, host, port, db, pools={}, **options):
        '''
        Fetch a redis conenction pool for the unique combination of host
        and port. Will create a new one if there isn't one already.
        '''
        key = (host, port, db)
        rval = pools.get(key)
        if not isinstance(rval, ConnectionPool):
            rval = ConnectionPool(host=host, port=port, db=db, **options)
            pools[key] = rval

        return rval