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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 wait_instances_running(ec2, instances):
 """ Wait until no instance in the given iterable is 'pending'. Yield every instance that entered the running state as soon as it does. :param boto.ec2.connection.EC2Connection ec2: the EC2 connection to use for making requests :param Iterator[Instance] instances: the instances to wait on :rtype: Iterator[Instance] """

    running_ids = set()
    other_ids = set()
    while True:
        pending_ids = set()
        for i in instances:
            if i.state == 'pending':
                pending_ids.add(i.id)
            elif i.state == 'running':
                assert i.id not in running_ids
                running_ids.add(i.id)
                yield i
            else:
                assert i.id not in other_ids
                other_ids.add(i.id)
                yield i
        log.info('%i instance(s) pending, %i running, %i other.',
                 *map(len, (pending_ids, running_ids, other_ids)))
        if not pending_ids:
            break
        seconds = max(a_short_time, min(len(pending_ids), 10 * a_short_time))
        log.info('Sleeping for %is', seconds)
        time.sleep(seconds)
        for attempt in retry_ec2():
            with attempt:
                instances = ec2.get_only_instances(list(pending_ids))