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();
}