'Generates _id_map from _alternatives to allow validating contents'

def _setup(self):

    cls = self.__class__
    cls._id_map = {}
    cls._name_map = {}
    for (index, info) in enumerate(cls._alternatives):
        if (len(info) < 3):
            info = (info + ({},))
            cls._alternatives[index] = info
        id_ = _build_id_tuple(info[2], info[1])
        cls._id_map[id_] = index
        cls._name_map[info[0]] = index
