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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 backup(fn):
 """If ``fn`` exists, rename it and issue a warning This function will rename an existing filename {fn}.bak.{i} where i is the smallest integer that gives a filename that doesn't exist. This naively uses a while loop to find such a filename, so there shouldn't be too many existing backups or performance will degrade. Parameters fn : str The filename to check. """

    if not os.path.exists(fn):
        return

    backnum = 1
    backfmt = "{fn}.bak.{backnum}"
    trial_fn = backfmt.format(fn=fn, backnum=backnum)
    while os.path.exists(trial_fn):
        backnum += 1
        trial_fn = backfmt.format(fn=fn, backnum=backnum)

    warnings.warn("{fn} exists. Moving it to {newfn}"
                  .format(fn=fn, newfn=trial_fn),
                  BackupWarning)
    shutil.move(fn, trial_fn)