                 [unowned self] in
                 let lock = ScopedLock(self)
 
-                println("move to next")
                 self._prepareNextAssetReaders()
 
             }

                startTime = kCMTimeZero
                _readers.append(assetreader)
            } else {
                NSLog("Failed to instantiate a AssetReaderFragment.")
            }
        }


    */
    func advanceToNextAsset() {
        if !_readers.isEmpty {
            // autoRemoveOutdatedAssetsが有効ならばwindow外の古いアセットを削除
            if self.autoRemoveOutdatedAssets {
                if let assetPos = self._getAssetPositionOf(0.0) {
                    println("\(0..<assetPos.index) is outdated.")
