I think CLOS doesn't suffer from the expression problem. Both alterations to the data model can be implemented as a "patch" to the program without editing any of the original code.
Gah, do you know if there is a better name for this "pattern matching vs polymorphism" duality then? I think its still important in its own right since multimethods and other solutions to the expression problem usually apply to a more global-ish scope, unlike pattern matching and functions since those are primitive constructs and naturally nested as deep as you want.
