this ensures predictable arguments for macro implementations:

    <list-group>
        {["foo", "bar", "baz"].map(item => {
            return <span>{item}</span>;
        })}
    </list-group>

    registerMacro("list-group", (_, ...items) => {
        return <ul>
            {items.map(item => {
                return <li>{item}</li>;
            })}
        </ul>;
    });

otherwise the list-group macro would have to flatten `items` itself