built-in shelf
#libvalue is a built-in shelf that starts out with one item for each public identifier built into
OmniMark. These values are used by OmniMark's built-in entity manager when an entity is not resolved using
the #library shelf. (This allows you to avoid having to write an external-text-entity rule.)
The default current-item is the last item on the shelf, as in programmer-defined shelves.