persistable sclass SubMatrix<A> extends AbstractMatrix<A> {
  Matrix<A> matrix;
  int x1, y1;

  *(Matrix<A> *matrix, Rect r) {
    super(r.w, r.h);
    x1 = r.x; y1 = r.y;
  }

  // currently no range checks
  
  public A get(int x, int y) { ret matrix.get(x-x1, y-y1); }
  public void set(int x, int y, A a) { matrix.set(x-x1, y-y1, a); }
}