persistable sclass ArrayMatrix<A> extends AbstractMatrix<A> {
  new BitSet bs;
  
  *(int *w, int *h) {}
  *(int *w, int *h, BitSet *bs) {}

  public A get(int x, int y) { ret bs.get(y*w+x); }
  public void set(int x, int y, A a) { bs.set(y*w+x, a); }
}