This talk is dedicated to another randomised testing technique. Unlike property-based testing, which inspects our src directory, mutation testing audits contents of the test one. It introduces subtle bugs to source code of our projects and verifies that our test suites catch those synthetic problems. Clojure, a homoiconic language with dynamic code reloading, should offer us an excellent foundation for building a mutation testing library. But there’s not a single one out there. Why? Let’s try to answer this question together.