anchor.diff:   */
   class FutureAlreadyCompleteException implements Exception {
     FutureAlreadyCompleteException() {}
  +  String toString() => "Exception: future already completed";
   }
   
   
  
positive.code:   void then(void onComplete(T value)) {
      if (hasValue) {
        onComplete(value);
      } else {
        _listeners.add(onComplete);
      }
    }
  
  
negative.code:  */
  class FutureNotCompleteException implements Exception {
    FutureNotCompleteException() {}
    String toString() => "Exception: future has not been completed";
  }
  
  /**
  