I contributed the regroup-keywords function to allow 'flattened' match syntax for :when and :as. It used gensym to make a marker to simplify the algorithm. After seeing how other people had done similar things with sentinel values, I realized that (Object.) is a better unique value. Theoretically, an evil user could use the same symbol that the gensym had created. Also, it's better to test with identical? rather than = since the sentinel is unique. I will attach a patch with a slight refactoring.