                 if let assetreader = AssetReaderFragment(asset:asset) {
                     self._readers.append(assetreader)
                 } else {
-                    NSLog("Failed to instantiate a AssetReaderFragment.")
+                    NSLog("0) Failed to instantiate a reader of [\((asset as AVURLAsset).URL.lastPathComponent!)]")
                 }
             }
         }

    */
    func advanceToNextAsset() {
        if !_readers.isEmpty {
            // 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assetPos.index) is outdated.")

                [unowned self] in
                let lock = ScopedLock(self)

                self._prepareNextAssetReaders()

            }
