http://clojure-doc.org/articles/language/concurrency_and_parallelism.html
http://grokbase.com/t/gg/clojure/12afy2cz9p/how-to-represent-trees-for-use-with-zippers
http://stackoverflow.com/questions/26950917/printing-vector-as-a-tree
http://clojure.org/concurrent_programming
https://github.com/scgilardi/slingshot
http://stackoverflow.com/questions/20708580/try-and-slingshot-try-differences
https://github.com/cgrand/enlive
https://gist.github.com/jhannes/948150
https://github.com/functional-koans/clojure-koans/tree/master/src/koans