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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 verify_proxy_ticket(ticket, service):
 """ Verifies CAS 2.0+ XML-based proxy ticket. :param: ticket :param: service Returns username on success and None on failure. """


    params = {'ticket': ticket, 'service': service}

    url = (urljoin(settings.CAS_SERVER_URL, 'proxyValidate') + '?' +
           urlencode(params))

    page = urlopen(url)

    try:
        response = page.read()
        tree = ElementTree.fromstring(response)
        if tree[0].tag.endswith('authenticationSuccess'):
            username = tree[0][0].text
            proxies = []
            if len(tree[0]) > 1:
                for element in tree[0][1]:
                    proxies.append(element.text)
            return {"username": username, "proxies": proxies}
        else:
            return None
    finally:
        page.close()