func export(withCompletion completion: ((URL?) -> Void)?) {
        var reversedAudioAssetTrack: AVAssetTrack? = nil
        var reversedVideoAssetTrack: AVAssetTrack? = nil
        
        separator.separateReversed { (audioUrl, videoUrl) in
            let composition: AVMutableComposition = AVMutableComposition()
            let videoCompositionTrack: AVMutableCompositionTrack = composition.addMutableTrack(withMediaType: AVMediaTypeVideo, preferredTrackID: kCMPersistentTrackID_Invalid)
            let audioCompositionTrack: AVMutableCompositionTrack = composition.addMutableTrack(withMediaType: AVMediaTypeAudio, preferredTrackID: kCMPersistentTrackID_Invalid)
            
            if let audio = audioUrl {
                let reversedAudioAsset = AVURLAsset(url: audio)
                reversedAudioAssetTrack = reversedAudioAsset.tracks(withMediaType: AVMediaTypeAudio).first
            }
//            let reversedAudioAsset = AVURLAsset(url: audioUrl!)
//            reversedAudioAssetTrack = reversedAudioAsset.tracks(withMediaType: AVMediaTypeAudio).first

            
            if let video = videoUrl {
                let reversedVideoAsset = AVURLAsset(url: video)
                reversedVideoAssetTrack = reversedVideoAsset.tracks(withMediaType: AVMediaTypeVideo).first
            }
//            let reversedVideoAsset = AVURLAsset(url: videoUrl!)
//            reversedVideoAssetTrack = reversedVideoAsset.tracks(withMediaType: AVMediaTypeVideo).first
            
            do {
                if let reversedVideoAssetTrack = reversedVideoAssetTrack {
                    try videoCompositionTrack.insertTimeRange(reversedVideoAssetTrack.timeRange, of: reversedVideoAssetTrack, at: kCMTimeZero)
                }
                if let reversedAudioAssetTrack = reversedAudioAssetTrack {
                    try audioCompositionTrack.insertTimeRange(reversedAudioAssetTrack.timeRange, of: reversedAudioAssetTrack, at: kCMTimeZero)
                }
            } catch let error {
                print("\(error)")
                return
            }
            
            let export: AVAssetExportSession? = AVAssetExportSession(asset: composition, presetName: AVAssetExportPresetHighestQuality)
            export?.outputURL = self.outputUrl
            export?.outputFileType = AVFileTypeQuickTimeMovie
            export?.shouldOptimizeForNetworkUse = true
            export?.exportAsynchronously(completionHandler: {
                if export?.status == .completed {
                    Swift.print(export?.outputURL ?? "nil")
                    export?.outputURL?.saveVideoToAlbum()
                    completion?(export?.outputURL)
                } else {
                    print("failed to export")
                    completion?(nil)
                }
            })
        }
    }