persistable sclass TargetAndActual<A> extends MetaWithChangeListeners {
  settableWithVar A target;
  settableWithVar A actual;
  
  *(A *target, A *actual) {}
  
  toString {
    ret commaCombine(
      target == null ?: "Target: " + target,
      actual == null ?: "Actual: " + actual);
  }
  
  A value() { ret actual; }
}