// implements BitMatrix
persistable sclass PointSetBitMatrix extends AbstractMatrix<Bool> {
  new Set<Pt> points;
  
  *(int *w, int *h) {}
  *(int *w, int *h, Set<Pt> *points) {}

  public Bool get(int x, int y) { ret points.contains(pt(x, y)); }
  public void set(int x, int y, Bool a) { points.add(pt(x, y)); }
  
  int nSetBits() { ret points.size(); }
}