static L xtok_joinBrackets3(L tok) {
  new L t;
  Map<Int, Int> map = getBracketMap(tok);
  for (int i = 0; i < l(tok); i++) {
    Int dest = map.get(i);
    if (dest != null && !(i == 1 && dest == l(tok)-2)) {
      t.add(cncAddFirstAndLast(subList(tok, i, dest+1)));
      i = dest;
    } else
      t.add(tok.get(i));
  }
  ret t;
}

Began life as a copy of #1008803