def schedule_url(year, stype, week):
    """
    Returns the NFL.com XML schedule URL. `year` should be an
    integer, `stype` should be one of the strings `PRE`, `REG` or
    `POST`, and `gsis_week` should be a value in the range
    `[0, 17]`.
    """
    xmlurl = 'http://www.nfl.com/ajax/scorestrip?'
    if stype == 'POST':
        week += 17
        if week == 21:  # NFL.com you so silly
            week += 1
    return '%sseason=%s&seasonType=%s&week=%s' % (xmlurl, year, stype, week)