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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 vtt_formatter(subtitles, padding_before=0, padding_after=0):
 """ Serialize a list of subtitles according to the VTT format, with optional time padding. """

    text = srt_formatter(subtitles, padding_before, padding_after)
    text = 'WEBVTT\n\n' + text.replace(',', '.')
    return text