The fact that the file compiler represents literal objects externally in a compiled file and must later reconstruct suitable equivalents of those objects when that file is loaded imposes a need for constraints on the nature of the objects that can be used as literal objects in code to be processed by the file compiler.
An object that can be used as a literal object in code to be processed by the file compiler is called an externalizable object .
We define that two objects are similar if they satisfy a two-place conceptual equivalence predicate (defined below), which is independent of the Lisp image so that the two objects in different Lisp images can be understood to be equivalent under this predicate. Further, by inspecting the definition of this conceptual predicate, the programmer can anticipate what aspects of an object are reliably preserved by file compilation.
The file compiler must cooperate with the loader in order to assure that in each case where an externalizable object is processed as a literal object, the loader will construct a similar object.
The set of objects that are externalizable objects are those for which the new conceptual term "similar" is defined, such that when a compiled file is loaded, an object can be constructed which can be shown to be similar to the original object which existed at the time the file compiler was operating.
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