sclass Lowest_withRunnerUp<A> extends Lowest<A> {
  A runnerUp;
  double runnerUpScore;

  bool put(A a, double score) {
    bool change = false;
    synchronized(this) {
      if (a != null && isNewBest(score)) {
        runnerUp = best;
        runnerUpScore = this.score;
        best = a;
        this.score = score;
        set change;
      }
    }
    if (change)
      pcallF(onChange);
    ret change;
  }
  
  synchronized Pair<A, Double> runnerUpPair() { ret runnerUp == null ? null : main.pair(runnerUp, runnerUpScore); }
}

Began life as a copy of #1006448