operator
true if the public identifier for the entity being tested is on the #library shelf, and false otherwise. Replacing is with isnt will reverse the results. (entity | notation) (is | isnt) in-library
Either an entity or a notation test used to determine whether the program contains, respectively, library rules for the specified entity, or a system identifier of the specified notation.
in-library succeeds only if the entity is an external entity, with a public identifier that is mapped to a system identifier in an OmniMark library declaration.