Function plan_smart 

pub fn plan_smart(
    lib: &Library,
    id: &str,
    dest: &Path,
    how: Collision,
    hidden: &HashSet<String>,
) -> Option<Plan>

Plans exporting the smart folder id as one flat directory under dest holding the originals of its items (Eagle exports a smart folder without its subfolders). Items in hidden folders are left out. None if there is no such smart folder.