'Returns (new_offset, object)'

@classmethod
def unpack_subclass(cls, spec, n_bits, raw, offset):

    assert (cls is not nx_learn_spec), 'Must    call    on    subclass'
    c = _flow_mod_spec_to_class(cls._is_src, spec)
    (offset, o) = c.unpack_new(n_bits, raw, offset)
    return (offset, o)
