Which lesser-known but useful features of Clojure do you find yourselves using? Feel free to share little tricks and idioms, but try to restrict yourselves to Core and Contrib. I found some really interesting information in answers to these similar questions: Hidden features of Haskell Hidden features of Python Hidden features of Java Hidden features of C There are many more "Hidden feature" questions for other languages, so I thought it would be nice to have one for Clojure, too.