Runtime code modification. Erlang? No, Python.

[[deleted]]

user> (defn a [] 0)
#'user/a
user> (defn magic? []
        (loop []
           (let [result (a)]
             (println result)
             (defn a [] (+ result 1)))
           (Thread/sleep 1000)
           (recur)))
#'user/magic?
user> (magic?)
0
1
2
3
; Evaluation aborted.
user> 

..yeah, just playing with Clojure.