gistfile1.clj

(defmacro extend-fn [name args & body]
  `(let [old-fn# (var-get (var ~name))
         new-fn# (fn [& p#] 
                   (let [~args p#] 
                     (do ~@body)))
         wrapper# (fn [& params#]
                    (if (= ~(count args) (count params#))
                      (apply new-fn# params#)
                      (apply old-fn# params#)))] 
     (alter-var-root (var ~name) (constantly wrapper#))))