code: def parse_selection(self, selection, name=None):
          
  
          if type(selection) is tuple:
              # range
              process = self._process_range
          elif selection.startswith('@'):
              # verbatim make_ndx command
              process = self._process_command
              selection = selection[1:]
          else:
              process = self._process_residue
          return process(selection, name)
docs: Retuns (groupname, filename) with index group.