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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 prettify(amount, separator=','):
 """Separate with predefined separator."""

    orig = str(amount)
    new = re.sub("^(-?\d+)(\d{3})", "\g<1>{0}\g<2>".format(separator), str(amount))
    if orig == new:
        return new
    else:
        return prettify(new)