code: def doc(self, export='plain'):
          
          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()
docs: 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