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
  
  abstract CharacterClassIterator acceptChar(char c);
  
  // query methods about next character
  
  abstract bool canEndHere();
  abstract bool canAcceptChar(char c);
  
  // 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();
}