sclass FieldVar<A> extends VarWithNotify<A> {
  IHasChangeListeners containingObject;
  S fieldName;
  IF0<A> getter;
  IVF1<A> setter;

  *(IHasChangeListeners *containingObject,
    S *fieldName, IF0<A> *getter, IVF1<A> *setter) {
    containingObject.onChangeAndNow(l0 _updateFromObject);
  }
  
  void _updateFromObject {
    set(getter!);
  }
  
  public void fireChange :: before {
    setter.get(get());
  }
  
  public selfType onChange(IVF1<A> r) {
    if (r != null) onChange(-> r.get(get()));
    this;
  }
}

Began life as a copy of #1034091