def clean_url(url):
    """ Tacks an s3:// onto the beginning of a URL if necessary. 

        url: an S3 URL, or what should be one

        Return value: S3 URL
    """
    if url[:6] == 's3n://':
        return ('s3://' + url[6:])
    elif url[:5] != 's3://':
        return ('s3://' + url)
    else:
        return url