sclass CCI_Substring extends CharacterClassIterator {
  S s;
  int i, l;
  CharacterClassIterator next;

  *() {}
  *(S *s, int start, int end) { i = start; l = end; }
  
  // no check because of method contract
  CharacterClassIterator acceptChar(char c) {
    ret ++i >= l && next != null ? next : this;
  }

  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));
  }
}