code: function(box) {
        var b = new c()
  
        // merge boxes
        b.x      = Math.min(this.x, box.x)
        b.y      = Math.min(this.y, box.y)
        b.width  = Math.max(this.x + this.width,  box.x + box.width)  - b.x
        b.height = Math.max(this.y + this.height, box.y + box.height) - b.y
  
        return fullBox(b)
      }
docs: Merge rect box with another, return a new instance