 
 extension Diskcached: ImageCache {
 
-    private var _concurrent_queue: dispatch_queue_t {
-        return dispatch_queue_create(nil, DISPATCH_QUEUE_CONCURRENT)
-    }
-
     subscript (aKey: NSURL) -> UIImage? {
 
         get {
             var value : UIImage?
-            dispatch_sync(_concurrent_queue) {
+            dispatch_sync(_subscript_queue) {
                 value = self.objectForKey(aKey)
             }
 

    }
    let directory = Directory()

    private let _queue = dispatch_queue_create("swift.imageloader.queues.diskcached", DISPATCH_QUEUE_SERIAL)
}

// MARK: accessor

            self.images[aKey] = nil
        }

        dispatch_async(_set_queue, block)
    }
}

