<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 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