def nt_to_tuple(nt):
    """Convert a namedtuple instance to a tuple.

    Even if the instance's __iter__ method has been changed.  Useful for
    writing derived instances of typeclasses.

    :param nt: namedtuple instance.

    :returns: A tuple containing each of the items in nt

    """
    return tuple(getattr(nt, f) for f in type(nt)._fields)