sclass Splitter2<A, B> {
  HashMap<A, MultiSet<B>> map = new HashMap;

  // may return null if unknown
  B get(A a) {
    MultiSet<B> ms = map.get(a);
    ret ms == null ? null : ms.getMostPopularEntry();
  }
  
  void learn(A a, B b) {
    MultiSet<B> ms = map.get(a);
    if (ms == null) map.put(a, ms = new MultiSet);
    ms.add(internIfString(b));
  }
  
  void unlearn(A a, B b) {
    MultiSet<B> ms = map.get(a);
    if (ms == null) ret;
    ms.remove(b);
    if (empty(ms)) map.remove(a);
  }
  
  void clear { map.clear(); }
}

Began life as a copy of #1009690