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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 export_fem(self, arms=None, format='json', df_kwargs=None):
 """ Export the project's form to event mapping Parameters arms : list Limit exported form event mappings to these arm numbers format : (``'json'``), ``'csv'``, ``'xml'`` Return the form event mappings in native objects, csv or xml, ``'df''`` will return a ``pandas.DataFrame`` df_kwargs : dict Passed to pandas.read_csv to control construction of returned DataFrame Returns ------- fem : list, str, ``pandas.DataFrame`` form-event mapping for the project """

        ret_format = format
        if format == 'df':
            from pandas import read_csv
            ret_format = 'csv'
        pl = self.__basepl('formEventMapping', format=ret_format)
        to_add = [arms]
        str_add = ['arms']
        for key, data in zip(str_add, to_add):
            if data:
                pl[key] = ','.join(data)
        response, _ = self._call_api(pl, 'exp_fem')
        if format in ('json', 'csv', 'xml'):
            return response
        elif format == 'df':
            if not df_kwargs:
                return read_csv(StringIO(response))
            else:
                return read_csv(StringIO(response), **df_kwargs)