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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 ExistingFileOrNone(fname):
 """Like `Existingfile`, but if `fname` is string "None" then return `None`."""

    if os.path.isfile(fname):
        return fname
    elif fname.lower() == 'none':
        return None
    else:
        raise ValueError("%s must specify a valid file name or 'None'" % fname)