// f is a function or an instance of Cache
sclass DependentCache<A> extends Cache<A> {
  O f, lastValue;
  
  *() {}
  *(O *maker, O *f) {}
  
  A get() {
    lock lock;
    O value;
    if (f instanceof Cache) value = f/Cache.changeCount;
    else {
      value = callF(f);
      if (value instanceof Cache) value = value/Cache.changeCount;
    }
    if (value != lastValue) {
      clear();
      value = lastValue;
    }
    ret super.get();
  }
}