opacity {
  child: fill {
    child: opacity {
      opacity: 0.4;
      child: color {
        bounds: 0 0 50 50;
        color: rgb(0,255,0);
      }
    }
    path: "\
M 10 0\
L 40 0\
L 40 10\
L 50 10\
L 50 40\
L 40 40\
L 40 50\
L 10 50\
L 10 40\
L 0 40\
L 0 10\
L 10 10\
Z";
    fill-rule: winding;
  }
}