sclass PointSetSimilarity {
  Set<Pt> set1, set2;
  int totalPoints;
  int sharedPoints;
  
  *(Set<Pt> *set1, Set<Pt> *set2) {
    for (Pt p : set1) {
      ++totalPoints;
      if (set2.contains(p))
        ++sharedPoints;
    }
    
    for (Pt p : set2)
      if (!set1.contains(p))
        ++totalPoints;
  }
  
  double get() {
    ret doubleRatio(sharedPoints, totalPoints);
  }
}