def cached(
            cls,
            release=MAX_ENSEMBL_RELEASE,
            species=human,
            server=ENSEMBL_FTP_SERVER):
        """
        Construct EnsemblRelease if it's never been made before, otherwise
        return an old instance.
        """
        init_args_tuple = cls.normalize_init_values(release, species, server)
        if init_args_tuple in cls._genome_cache:
            genome = cls._genome_cache[init_args_tuple]
        else:
            genome = cls._genome_cache[init_args_tuple] = cls(*init_args_tuple)
        return genome
Construct EnsemblRelease if it's never been made before, otherwise
        return an old instance.