ANSI Common Lisp 3 Evaluation and Compilation 3.2 Compilation 3.2.4 Literal Objects in Compiled Files
18.104.22.168 Extensions to Similarity Rules
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.