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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 utf8(value):

 """Converts a string argument to a byte string.
 
 If the argument is already a byte string or None, it is returned unchanged.
 Otherwise it must be a unicode string and is encoded as utf8.
 """

    if isinstance(value, _UTF8_TYPES):
        return value
    elif isinstance(value, unicode_type):
        return value.encode("utf-8")
    else:
        return str(value)