'Parse the header.  May return None if it cannot parse.'

@classmethod
def parse(cls, value):

    m = _rx_content_range.match((value or ''))
    if (not m):
        return None
    (s, e, l) = m.groups()
    if s:
        s = int(s)
        e = (int(e) + 1)
    l = (l and int(l))
    if (not _is_content_range_valid(s, e, l, response=True)):
        return None
    return cls(s, e, l)
