sclass LazyValueTreeMap<A, B> extends TreeMap<A, B> {
  static O placeholder = new O; // fool type system
  O valueMaker;
  
  *(Collection<A> keys, O *valueMaker) {
    for (A key : unnull(keys)) put(key, (B) placeholder);
  }
  
  public B get(O key) {
    B value = super.get(key);
    if (value == placeholder)
      put((A) key, value = (B) callF(valueMaker, key));
    ret value;
  }
  
  int numPlaceholders() {
    ret countOccurrences_id(values(), placeholder);
  }
  
  // revert to identity hashcode
  public bool equals(O o) { ret this == o; }
  public int hashCode() { ret systemHashCode(this); }
}