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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 get_linked_version(doi):
 """ Get the original link behind the DOI. :param doi: A canonical DOI. :returns: The canonical URL behind the DOI, or ``None``. 'http://stacks.iop.org/0295-5075/111/i=4/a=40005?key=crossref.9ad851948a976ecdf216d4929b0b6f01' """

    try:
        request = requests.head(to_url(doi))
        return request.headers.get("location")
    except RequestException:
        return None