Sorry for being confusing, it was intended as an illustration of why Clojure's approach is good. Non-immutable data being shared around is basically a recipe f

Sorry for being confusing, it was intended as an illustration of why Clojure's approach is good. Non-immutable data being shared around is basically a recipe for weird action at a distance, is the point. In what was intended to be a "compatible API, just change the imports" update, code broke because an unchecked invariant became a checked one, and then broke again because the workaround mutated data that was being referred to somewhere else.