abstract sclass CharacterClassIterator {

  // move method (go to next character)

  // callee can assume that c is compatible with canAcceptChar

  // return value can be this or follow-up iterator, but not null.

  // Caller must, after this call, use the return value instead of the old iterator.

  abstract CharacterClassIterator acceptChar(char c);

  // query methods about next character

  abstract bool canEndHere();

  bool canAcceptChar(char c) { ret contains(acceptedChars(), c); }

  // can be null

  abstract Collection<Char> acceptedChars();

  // This character range must contain all the characters that

  // are accepted by canAcceptChar at the current position.

  // character range should be interpreted in a way compatible with an agreed-on char comparator.

  // Method can return null if no chars are accepted at this position.

  abstract CharRange acceptedCharRange();

  // clone this iterator - may return same object if iterator is immutable

  abstract CharacterClassIterator cloneCCI();

}