A Logos persistent vector: [1 2 3].

Backed by Erlang's built-in :array module rather than a hand-rolled bit-partitioned trie or an external library: :array already gives O(log n) get/set with structural sharing on "mutation" (every update returns a new array, the old one is untouched, exactly the persistence a Lisp vector needs), ships with the BEAM (zero extra dependency), and is a well-worn, boring piece of the standard library rather than a new trie implementation Logos would have to get right and maintain itself. A real Clojure-style 32-way trie would beat it on constant factors, but that's a performance concern for later, not something correctness depends on today.