docs: def save(self, mode='all'):
          """Saves to the file specified by :attr:`~SR850.filename`.
  
          :param mode: Defines what to save.
  
              =======  ================================================
              Value    Description
              =======  ================================================
              'all'    Saves the active display's data trace, the trace
                       definition and the instrument state.
              'data'   Saves the active display's data trace.
              'state'  Saves the instrument state.
              =======  ================================================
  
          """
          if mode == 'all':
              self._write('SDAT')
          elif mode == 'data':
              self._write('SASC')
          elif mode=='state':
              self._write('SSET')
          else:
              raise ValueError('Invalid save mode.')