Clojure - Transient Data Structures

If a tree falls in the woods, does it make a sound? If a pure function mutates some local data in order to produce an immutable return value, is that ok? It’s an interesting question. Clojure data structures use mutation every time you call, e.g. assoc, creating one or more arrays and mutating them, before returning them for immutable use thereafter. The reason is performance - you simply can’t ge