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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def download(directory, youtube_id, clear=False):
 """Download the audio of a YouTube video. The audio is downloaded in the highest available quality. Progress is printed to `stdout`. The file is named `youtube_id.m4a`, where `youtube_id` is the 11-character code identifiying the YouTube video (can be determined from the URL). Parameters directory : str The directory in which to save the downloaded audio file. youtube_id : str 11-character video ID (taken from YouTube URL) clear : bool If `True`, it deletes the downloaded video. Otherwise it downloads it. Defaults to `False`. """

    filepath = os.path.join(directory, '{}.m4a'.format(youtube_id))
    if clear:
        os.remove(filepath)
        return
    if not PAFY_AVAILABLE:
        raise ImportError("pafy is required to download YouTube videos")
    url = 'https://www.youtube.com/watch?v={}'.format(youtube_id)
    video = pafy.new(url)
    audio = video.getbestaudio()
    audio.download(quiet=False, filepath=filepath)