sS reversedSubstring(S s, int x, int y) {
  if (s == null) null;
  if (x < 0) x = 0;
  int n = s.length();
  if (y < x) y = x;
  if (y > n) y = n;
  if (x >= y) ret "";
  
  // TODO: Is this actually faster than reversed(substring(...))?
  char[] chars = new[y-x];
  int j = y-x;
  for (int i = x; i < y; i++)
    chars[--j] = s.charAt(i);
  ret String.valueOf(chars);
}