datastore.Cursor - paging in reverse supported?

I'm looking at the documentation for using cursors. Are these forward-only cursors - we can't page backwards?:

http://code.goo开发者_开发技巧gle.com/appengine/docs/java/datastore/queriesandindexes.html#Query_Cursors

Cursor cursor = Cursor.fromWebSafeString(cursorString);
Map<String, Object> extensionMap = new HashMap<String, Object>();
extensionMap.put(JDOCursorHelper.CURSOR_EXTENSION, cursor);
query.setExtensions(extensionMap);
query.setRange(0, 20);

could we set the range to (-20, 0) ?

I imagine the user would want to be able to go to the previous page of elements as well as forward.

No, there is no way to go backwards with cursors with the current release.

You could "fake" it, however, by caching previous cursors.

Note: According to Alfred Fuller's Google IO talk, at some point in the future app engine will hopefully support reverse cursors too.