static CharacterClassIterator stringToCharacterClassIterator(S s) {
  ret new stringToCharacterClassIterator_C(s);
}
  
sclass stringToCharacterClassIterator_C extends CharacterClassIterator {
  S s;
  int i, l;
  
  *() {}
  *(S *s) { l = l(s); }
  
  // no check because of method contract
  void acceptChar(char c) {
    ++i;
  }

  bool canEndHere() {
    ret i >= l;
  }
  
  bool canAcceptChar(char c) {
    ret i < l && s.charAt(i) == c;
  }

  CharRange acceptedCharRange() {
    ret i >= l ? null : singletonCharRange(s.charAt(i));
  }
}