static L<S> tok_withoutOuterBrackets(L<S> tok) {
  if (!eq(get(tok, 1), "(") || !eq(nextToLast(tok), ")")) ret tok;
  int level = 1;
  for (int i = 3; i < l(tok)-2; i += 2) {
    S t = get(tok, i);
    if (eq(t, "(")) ++level;
    else if (eq(t, ")")) if (--level == 0) ret tok;
  }
  ret level == 1 ? subList(tok, 2, l(tok)-2) : tok;
}