svoid imageSurfaceOnLeftMouseDown(ImageSurface is, IVF1<Pt> action) {
  if (is == null || action == null) ret;
  swing {
    MouseAdapter ma = new MouseAdapter {
      public void mousePressed(MouseEvent e) {
        if (e.getButton() == 1)
          pcallF(action, is.pointFromEvent(e));
      }
    };
    is.addMouseListener(ma);
  }
}