code: function whitespace(layout) {
  	var whitespace = layout.size[0] * layout.size[1]
  	for (var i = 0; i < layout.boxes.length; i++) {
  		var box = layout.boxes[i]
  		whitespace -= box.size[0] * box.size[1]
  	}
  	return whitespace
  }
docs: determines the amount of whitespace area remaining in `layout`