// scheduler isn't actually used, but might be
// (e.g. for collection stats)
sclass PMapCollector<A, B> extends Probabilistic {
  new Map<A, B> map;
  new L<IVF2<A, B>> onNewEntry;
  new L<IVF3<A, B, B>> onChangedEntry;
  
  void put(A key, B value) {
    bool isNew = !map.containsKey(key);
    B oldValue = null;
    bool isChanged = !isNew && !eq(oldValue = map.get(key), value);
    if (isNew || isChanged)
      map.put(key, value);
    if (isNew)
      pcallFAll(onNewEntry, key, value);
    else
      pcallFAll(onChangedEntry, key, oldValue, value);
  }
  
  void onNewEntry
}