sclass LazyValueTreeMap<A, B> extends TreeMap<A, B> {
  static new O placeholder;
  O valueMaker;
  
  LazyValueTreeMap(Collection<A> keys, O *valueMaker) {
    for (A key : unnull(keys)) put(key, placeholder);
  }
  
  public get(O key) {
    B value = super.get(key);
    if (value == placeholder)
      put(key, value = callF(valueMaker, key));
    ret value;
  }
}