@unknown_lookup.setter<EOL><INDENT>def unknown_lookup(self, unknown_lookup):<DEDENT>
self._unknown_lookup = unknown_lookup<EOL>
Vector lookup for unknown tokens.

        If not None, unknown_lookup[tokens] is called for any unknown tokens.

        Parameters
        ----------
        unknown_lookup : Mapping[List[str], nd.NDarray]
            Vector lookup mapping from tokens to vectors.