Function plan_folder 

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

Plans exporting the folder id as a directory tree under dest: a directory per folder and subfolder, each holding the originals of the live items filed directly in it (an item in two folders is copied into both). Items in any folder of hidden (locked ones) are left out, as are hidden subfolders. Sibling folders whose names clean up to the same directory share it. None if there is no such folder.