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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 flipwritable(fn, mode=None):
 """ Flip the writability of a file and return the old mode. Returns None if the file is already writable. """

    if os.access(fn, os.W_OK):
        return None
    old_mode = os.stat(fn).st_mode
    os.chmod(fn, stat.S_IWRITE | old_mode)
    return old_mode