;;;; v4 redefines only [added] — itself introduced at run time by v3, so it
;;;; lives in the registry and not in any symbol table.
;;;;
;;;; This is the case that separates a real implementation from a plausible
;;;; one. v3's [bump] is already installed and is not rebuilt here, so it picks
;;;; this up only if its call to [added] goes through a *cell* that both
;;;; modules found by name. Had v3 cached the address of the function instead
;;;; of the address of its cell, everything else would still pass and this
;;;; would silently keep running v3's [added].
(defvar counter i64)
(defvar extra i64)

(defn helper [x i64] i64 (* x 2))

(defn added [] i64
  (set extra (+ extra 100))
  extra)

(defn bump [] i64
  (println "v3")
  (set counter (+ counter (added)))
  (helper counter))

(defn outer [] i64 (bump))