I haven't looked at the source, but the avoidance of full copy-on-write, laziness and instrumentation of immutable structures when returning from functions that

I haven't looked at the source, but the avoidance of full copy-on-write, laziness and instrumentation of immutable structures when returning from functions that update / assoc them will be non-trivial to get right. Not even to mention software transactional memory Clojure supports at all.