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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_cookie_header(jar, request):
 """ Produce an appropriate Cookie header string to be sent with `request`, or None. :rtype: str """

    r = MockRequest(request)
    jar.add_cookie_header(r)
    return r.get_new_headers().get('Cookie')