     private var trackRect = CGRectZero
     private let calipers:[UIView]
     private let tracks:[UIView]
-    private let calipersAndTracks:[UIView]
     
     //MARK: - Setup Etc.
     
     required init(coder aDecoder: NSCoder) {
         calipers = [leftCaliper, rightCaliper]
         tracks = [leftTrack, rightTrack]
-        calipersAndTracks = calipers + tracks
         super.init(coder: aDecoder)
     }
     

    
    override func beginTrackingWithTouch(touch: UITouch, withEvent event: UIEvent) -> Bool {
        resetCaliperRects()
        UIView.animateWithDuration(0.2, animations: { self.calipersAndTracks.map({ $0.alpha = 1 }) })
        return super.beginTrackingWithTouch(touch, withEvent: event)
    }
    

    private func finishTracking() {
        UIView.animateWithDuration(0.4, animations: { () -> Void in
            self.resetCaliperRects()
            (self.calipers + self.tracks).map({ $0.alpha = 0 })
        })
    }
}
