// as long as you don't access the by-numeral-index functions,
// you can add and remove elements quickly.
// upon accessing the list, it is updated once to fit the new data
sclass ProbabilisticList<A> extends AbstractRandomAccessList<WithProbability<A>> {
  // these are the 2 main data structures we update
  Map<A, Double> probabilities = hashMap();
  MultiSetMap<Double, A> byProbability = multiSetMap_outerDescTreeMap_innerLinkedHashSet();
  
  L<WithProbability<A>> renderedAsList; // this is only updated when requested
  
  public WithProbability<A> get(int idx) {
    if (idx < 0 || idx >= size()) null;
    makeListUpToIndex(idx);
    ret renderedAsList.get(idx);
  }
  
  public int size() { ret l(probabilities); }
  
  void makeListUpToIndex(int idx) {
    if (idx < l(renderedAsList)) ret;
    if (nempty(renderedAsList)) {
      A last = last(renderedAsList);
      double probability = probabilites.get(last);
      Set<> = byProbability.get(probability)
    while ( <= idx)
      byProbability.get(
  }
}