// doesn't implement all methods yet
sclass ValueOnDemandMap<A, B> extends AbstractMap<A, B> {
  Set<A> keys;
  IF1<A, B> makeValue;
  
  *() {}
  *(Set<A> *keys, IF1<A, B> *makeValue) {}
  
  public B get(O a) {
    ret !containsKey(a) ? null : makeValue.get(a);
  }
  
  public bool containsKey(O a) {
    ret keys.contains(a);
  }
  
  public Set<Map.Entry<A, B>> entrySet() {
    ret map.entrySet(); // TODO
  }
  
  public Set<A> keySet() { ret keys; }
  
  public int size() { ret keys.size(); }
}