def _insert(self, bs, pos):
        """Insert bs at pos."""
        assert 0 <= pos <= self.len
        if pos > self.len // 2:
            # Inserting nearer end, so cut off end.
            end = self._slice(pos, self.len)
            self._truncateend(self.len - pos)
            self._append(bs)
            self._append(end)
        else:
            # Inserting nearer start, so cut off start.
            start = self._slice(0, pos)
            self._truncatestart(pos)
            self._prepend(bs)
            self._prepend(start)
        try:
            self._pos = pos + bs.len
        except AttributeError:
            pass
        assert self._assertsanity()
Insert bs at pos.