Security-minded folks might be interested in the "clojail" sandboxing library (<a href="https://github.com/cognitivedissonance/clojail/blob/master/src/clojail/testers.clj#L9" rel="nofollow">https://github.com/cognitivedissonance/clojail/blob/master/s...</a>) that this webapp uses (<a href="https://github.com/dbyrne/4clojure/blob/74f381701b62a12e2bfb7047e67c4a0e8d064221/src/foreclojure/problems.clj#L35" rel="nofollow">https://github.com/dbyrne/4clojure/blob/74f381701b62a12e2bfb...</a>). clojail's README contains some caveats worth reading.
I thought this site was very cool, but some of the problems could be explained more clearly. I was getting things without any idea why my answers were incorrect.