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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 format_stack(skip=0, length=6, _sep=os.path.sep):
 """ Returns a one-line string with the current callstack. """

    return ' < '.join("%s:%s:%s" % (
        '/'.join(f.f_code.co_filename.split(_sep)[-2:]),
        f.f_lineno,
        f.f_code.co_name
    ) for f in islice(frame_iterator(sys._getframe(1 + skip)), length))