static join(...actionLists) {
        let out = [];
        for (const list of actionLists) {
            if (!list.length) {
                // skip
            }
            else if (out.length) {
                out = [...out, new Separator(), ...list];
            }
            else {
                out = list;
            }
        }
        return out;
    }
Joins all non-empty lists of actions with separators.