'upper' library shelf maker

If you have too few book in a category to use this
https://www.thingiverse.com/thing:3598004
consecutively, you can use this tag thing to alternate between upper and lower signs.
In the photo you see the upper and the lower version.

There are more files at https://github.com/benjaminwand/OpenScad-random/tree/main/items/library-shelf-markers