// credits to stackoverflow.com/users/1442960/marcus
static S lexicographicallyNextString(S input) {
  final int lastCharPosition = input.length()-1;
  String inputWithoutLastChar = input.substring(0, lastCharPosition);
  char lastChar = input.charAt(lastCharPosition);
  char incrementedLastChar = (char) (lastChar + 1);
  // Handle int/char overflow.  This wasn't done above.
  if (incrementedLastChar == ((char) 0)) return input+incrementedLastChar;
  return inputWithoutLastChar+incrementedLastChar;
}