Given a fully qualified XML name, find a prefix

        e.g. {http://ns.adobe.com/pdf/1.3/}Producer -> pdf:Producer

def _prefix_from_uri(self, uriname):
        """Given a fully qualified XML name, find a prefix

        e.g. {http://ns.adobe.com/pdf/1.3/}Producer -> pdf:Producer
        """
        uripart, tag = uriname.split('}', maxsplit=1)
        uri = uripart.replace('{', '')
        return self.REVERSE_NS[uri] + ':' + tag