sclass LowestDouble {
  double best = Double.NaN;
  transient O onChange;
  
  synchronized bool isNewBest(double d) {
    ret isNaN(best) || d < best;
  }
  
  // may return NaN
  double get() {
    ret best;
  }
  
  bool put(double d) {
    bool change;
    synchronized(this) {
      if (isNaN(best) || d < best) {
        best = d;
        set change;
      }
    }
    if (change)
      pcallF(onChange);
    ret change;
  }
  
  synchronized void clear() { best = Double.NaN; }
  
  synchronized bool has() { ret !isNaN(best); }

  synchronized toString { ret has() ? str(best) : "-"; }
}

Began life as a copy of #1006448