interface IVarWithNotify<A> extends IVar<A>, IF0WithChangeListeners<A> {
  default selfType onChange(IVF1<A> r) {
    if (r == null) this;
    onChange(-> r.get(get()));
    this;
  }
  
  default selfType onChangeAndNow(IVF1<A> r) {
    if (r == null) this;
    onChangeAndNow(-> r.get(get()));
    this;
  }
}

Began life as a copy of #1009536