// uses identity to compare a and new value
static class WeakAssoc<A, B> {
  O f; // func(A) -> B
  WeakReference<A> a;
  B b;
  
  *() {}
  *(O *f) {}

  synchronized B get(A a) {
    if (a != (this.a != null ? this.a.get() : null)) {
      b = (B) callF(f, a);
      this.a = new WeakReference(a);
    }
    ret b;
  }
}