// assumes s is sorted
static S formatCharRange(S s) {
  new L<S> l;
  for (int i = 0; i < l(s); ) {
    int j = i+1;
    while (j < l(s) && s.charAt(j) == (char) (((int) s.charAt(j-1))+1)) ++j;
    if (j == i+1)
      l.add(S.format("%X", (int) s.charAt(i)));
    else
      l.add(S.format("%X", (int) s.charAt(i)) + "-" + S.format("%X", (int) s.charAt(j-1)));
    i = j;
  }
  ret join(" ", l);
}