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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 process_nxml_str(nxml_str, citation=None, offline=False, output_fname=default_output_fname):
 """Return a ReachProcessor by processing the given NXML string. NXML is the format used by PubmedCentral for papers in the open access subset. Parameters nxml_str : str The NXML string to be processed. citation : Optional[str] A PubMed ID passed to be used in the evidence for the extracted INDRA Statements. Default: None offline : Optional[bool] If set to True, the REACH system is ran offline. Otherwise (by default) the web service is called. Default: False output_fname : Optional[str] The file to output the REACH JSON output to. Defaults to reach_output.json in current working directory. Returns ------- rp : ReachProcessor A ReachProcessor containing the extracted INDRA Statements in rp.statements. """

    if offline:
        if not try_offline:
            logger.error('Offline reading is not available.')
            return None
        try:
            api_ruler = reach_reader.get_api_ruler()
        except ReachOfflineReadingError as e:
            logger.error(e)
            logger.error('Cannot read offline because the REACH ApiRuler '
                         'could not be instantiated.')
            return None
        try:
            result_map = api_ruler.annotateNxml(nxml_str, 'fries')
        except JavaException as e:
            logger.error('Could not process NXML.')
            logger.error(e)
            return None
        # REACH version < 1.3.3
        json_str = result_map.get('resultJson')
        if not json_str:
            # REACH version >= 1.3.3
            json_str = result_map.get('result')
        if json_str is None:
            logger.warning('No results retrieved')
            return None
        if isinstance(json_str, bytes):
            json_str = json_str.decode('utf-8')
        return process_json_str(json_str, citation)
    else:
        data = {'nxml': nxml_str}
        try:
            res = requests.post(reach_nxml_url, data)
        except requests.exceptions.RequestException as e:
            logger.error('Could not connect to REACH service:')
            logger.error(e)
            return None
        if res.status_code != 200:
            logger.error('Could not process NXML via REACH service.'
                         + 'Status code: %d' % res.status_code)
            return None
        json_str = res.text

        with open(output_fname, 'wb') as fh:
            fh.write(json_str.encode('utf-8'))
        return process_json_str(json_str, citation)