   folder MUST NOT be listed as an item in its parent folder.

    Also, since folders exist automatically, PUT and DELETE requests
    only need to be made to documents, and never to folders. A document
    PUT will make all ancestor folders along its path become non-empty;
    deleting the last document from a subtree will make that whole
    subtree become empty. Folders will therefore show up in their parent
    folder descriptions if and only if their subtree contains at least
    one document.

    A 