static Rect floodFillGrid_step(BitSet grid, int gw, int gh, int x, int y) {
  Rect r = null;
  new L<Pt> stack;
  stack.add(new Pt(x, y));
  while (nempty(stack)) {
    Pt p = popLast(stack);
    x = p.x;
    y = p.y;

    if (!(x < 0 || y < 0 || x >= gw || y >= gh)) {
      int idx = y*gw+x;
      
      if (grid.get(idx)) {
        grid.clear(idx);
        Rect me = new Rect(x, y, 1, 1);
        if (r == null)
          r = me;
        else
          r = rectUnion(r, me);

        stack.add(new Pt(x-1, y));
        stack.add(new Pt(x+1, y));
        stack.add(new Pt(x, y-1));
        stack.add(new Pt(x, y+1));
      }
    }
  }
  ret r;
}