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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 _prep(e):
 """ Normalizes lastupdate to a timestamp, and constructs a URL from the embedded attributes. """

        if 'lastupdate' in e:
            e['lastupdate'] = datetime.datetime.fromtimestamp(int(e['lastupdate']))
        for k in ['farm', 'server', 'id', 'secret']:
            if not k in e:
                return e
        e["url"] = "https://farm%s.staticflickr.com/%s/%s_%s_b.jpg" % (e["farm"], e["server"], e["id"], e["secret"])
        return e