sclass Best<A> {
  A best;
  double score;
  transient O onChange;
  
  synchronized bool isNewBest(double score) {
    ret best == null || !isNaN(score) && score > this.score;
  }
  
  synchronized double bestScore() {
    ret best == null ? Double.NaN : score;
  }
  
  double score() { ret bestScore(); }
  double getScore() { ret bestScore(); }
  
  bool put(A a, double score) {
    bool change = false;
    synchronized(this) {
      if (a != null && isNewBest(score)) {
        best = a;
        this.score = score;
        change = true;
      }
      change = true;
    }
    if (change)
      pcallF(onChange);
    ret change;
  }
  
  synchronized A get() { ret best; }
  synchronized bool has() { ret best != null; }
  
  synchronized Pair<A, Double> pair() { ret main.pair(best, bestScore()); }
  
  synchronized A getIfScoreAbove(double x) { ret score() >= x ? best : null; }
}