How about adding something to the *features* list to indicate the version of CLX loaded? Say, :clx-v11-r3, :clx-v11-r4, etc. I believe there are a very few minor source-incompatible changes in R4 CLX; we can also anticipate a few more of these when CLX is officially standardized. The *features* would help people that want to produce products that work with multiple versions.