abstract sclass CharacterClassIterator {
  // move method (go to next character)
  
  abstract void acceptChar(char c);
  
  // query methods about next character
  
  abstract bool canEndHere();
  abstract bool canAcceptChar(char c);
  
  // together, the following 2 methods define a inclusive character range.
  // 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.
  
  abstract char minCharAccepted();
  abstract char maxCharAccepted();
}