With Christophe Rhode's expert help, we found the spot where Hyperobject was failing with the latest CVS versions of CMUCL. I've been able to work around this CMUCL bug by temporarily removing a feature in Hyperobject that UMLisp is not currently using.
This work-around is great because I've been wanting to try the UMLisp benchmark with the MOP enhancements that Gerd Moellmann has added in the post-18e CMUCL release. The preliminary benchmarks show a significant improvement in the new version of CMUCL. I'll post a full set of benchmarks after the CMUCL bug gets fixed.
Thanks, Christophe!
