@classmethod
def from_bytes(cls, witver, witprog):
    """Instantiate from witver and data"""
    if not 0 <= witver <= 16:
        raise ValueError(
            'witver must be in range 0 to 16 inclusive; got %d' % witver)
    self = bytes.__new__(cls, witprog)
    self.witver = witver
    return self
