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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 doc(self, export='plain'):
 """ Build help document into a Texttable table :param ('plain', 'latex') export: export format :param save: save to file ``help_model.extension`` or not :param writemode: file write mode :return: None """

        title = '<{}.{}>'.format(self._group, self._name)
        table = Tab(export=export, title=title, descr=self.__doc__)
        rows = []
        keys = sorted(self._data.keys())
        for key in keys:
            val = self._data[key]
            suf = ''
            if key in self._mandatory:
                suf = ' *'

            elif key in self._powers + \
                    self._voltages + \
                    self._currents + \
                    self._z + \
                    self._y + \
                    self._dccurrents + \
                    self._dcvoltages + \
                    self._r + \
                    self._g + \
                    self._times:
                suf = ' #'

            c1 = key + suf
            c2 = self._descr.get(key, '')
            c3 = val
            c4 = self._units.get(key, '-')
            rows.append([c1, c2, c3, c4])

        table.add_rows(rows, header=False)
        table.header(['Parameter', 'Description', 'Default', 'Unit'])

        if export == 'plain':
            pass
        elif export == 'latex':
            raise NotImplementedError('LaTex output not implemented')

        return table.draw()