docs: def _get_at_pos(self, pos):
          """Return a widget for the line number passed."""
  
          if pos < 0:
              # line 0 is the start of the file, no more above
              return None, None
  
          if len(self.lines) > pos:
              # we have that line so return it
              return self.lines[pos], pos
  
          if self.file is None:
              # file is closed, so there are no more lines
              return None, None
  
          assert pos == len(self.lines), "out of order request?"
  
          self.read_next_line()
  
          return self.lines[-1], pos