SCM before = scm_hash_ref (id_to_group_hashtab_, before_id, SCM_BOOL_F);
SCM after = scm_hash_ref (id_to_group_hashtab_, after_id, SCM_BOOL_F);
SCM before = scm_hash_ref (id_to_group_hashtab_, before_id, SCM_BOOL_F);
SCM after = scm_hash_ref (id_to_group_hashtab_, after_id, SCM_BOOL_F);