<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 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