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) {
    try {
      ret matrix.get(x+x1, y+y1);
    } catch e {
      throw augmentException(e, this);
    }
  }
  
  public void set(int x, int y, A a) { matrix.set(x+x1, y+y1, a); }
  
  toString { ret renderFunctionCall("SubMatrix", x1, y1, w, h); }
}