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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_first_urn(self, urn):
 """ Provisional route for GetFirstUrn request :param urn: URN to filter the resource :param inv: Inventory Identifier :return: GetFirstUrn response """

        urn = URN(urn)
        subreference = None
        textId = urn.upTo(URN.NO_PASSAGE)
        if urn.reference is not None:
            subreference = str(urn.reference)
        firstId = self.resolver.getTextualNode(textId=textId, subreference=subreference).firstId
        r = render_template(
            "cts/GetFirstUrn.xml",
            firstId=firstId,
            full_urn=textId,
            request_urn=str(urn)
        )
        return r, 200, {"content-type": "application/xml"}