mask {
  source: container {
    color {
      color: red;
      bounds: 0 0 50 50;
    }
    color {
      color: yellow;
      bounds: 50 0 50 50;
    }
    color {
      color: magenta;
      bounds: 0 50 100 50;
    }
  }
  mask: color {
    color: black;
    bounds: 20 20 50 50;
  }
}