set_once(group_key, group_id, prop, to, modifiers, callback) {
        const identifiers = {$group_key: group_key, $group_id: group_id};
        this._set(prop, to, modifiers, callback, {identifiers, set_once: true});
    }
groups.set_once(group_key, group_id, prop, to, modifiers, callback)
        ---
        The same as groups.set, but adds a property value to a group only if it has not been set before.