Some objects, such as streams, readtables, and methods are not externalizable objects under the definition of similarity given above. That is, such objects may not portably appear as literal objects in code to be processed by the file compiler.
An implementation is permitted to extend the rules of similarity, so that other kinds of objects are externalizable objects for that implementation.
If for some kind of object, similarity is neither defined by this specification nor by the implementation, then the file compiler must signal an error upon encountering such an object as a literal constant.