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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 iso_639_alpha3(code):
 """Convert a given language identifier into an ISO 639 Part 2 code, such as "eng" or "deu". This will accept language codes in the two- or three- letter format, and some language names. If the given string cannot be converted, ``None`` will be returned. """

    code = normalize_code(code)
    code = ISO3_MAP.get(code, code)
    if code in ISO3_ALL:
        return code