// get() works as expected (calculating the element on demand).
// containsKey(), entrySet(), size() etc. only refer to the elements
// already made.
// Also allows put().
// Not synchronized
sclass AutoMap<A, B> extends AbstractMap<A, B> {
  Map<A, B> map = new HashMap;
  IF1<A, B> f;
  
  *() {}
  *(IF1<A, B> *f) {}
  *(IF0<B> f) { this.f = if0ToIF1(f); }
  *(Class<B> c) { this.f = a -> nu(c); }
  
  public B get(O a) {
    if (map.containsKey(a))
      ret map.get(a);
    cast a to A;
    B b = f.get(a);
    map.put(a, b);
    ret b;
  }
  
  // unclear semantics
  public bool containsKey(O a) {
    ret map.containsKey(a);
  }
  
  public Set<Map.Entry<A, B>> entrySet() {
    ret map.entrySet();
  }
  
  public int size() { ret map.size(); }
  
  public B put(A a, B b) {
    ret map.put(a, b);
  }
}