diff --git a/src/sci/script_tag.cljs b/src/sci/script_tag.cljs
index 70703a9..e22dd79 100644
--- a/src/sci/script_tag.cljs
+++ b/src/sci/script_tag.cljs
@@ -1,24 +1,27 @@
 (ns sci.script-tag
-  (:refer-clojure :exclude [defn])
+  (:refer-clojure :exclude [defn time])
   (:require [clojure.core :as c]
             [clojure.string :as str]
             [goog.object :as gobject]
             [goog.string]
             [sci.core :as sci]))
 
-(c/defmacro defn [fn-name & args]
-  (let [ns-sym (gensym "ns")]
-    `(let [~ns-sym (ns-name *ns*)]
-       (clojure.core/defn ~fn-name ~@args)
-       ~(when (:export (meta fn-name))
-          `(sci.script-tag/-export ~fn-name (str ~ns-sym "." '~fn-name))))))
+(c/defmacro time
+  "Evaluates expr and prints the time it took. Returns the value of expr."
+  [expr]
+  `(let [start# (cljs.core/system-time)
+         ret# ~expr]
+     (prn (cljs.core/str "Elapsed time: "
+                         (.toFixed (- (system-time) start#) 6)
+                         " msecs"))
+     ret#))
 
-(c/defn -export [f k]
+(c/defn export [k f]
   (let [k (munge k)
         parts (str/split k #"\.")]
     (loop [parts parts
            prev js/window]
-      (let [fpart (first parts)]
+      (when-first [fpart parts]
         (cond (= "user" fpart)
               (recur (rest parts) prev)
               (= 1 (count parts))
@@ -29,17 +32,19 @@
                 (let [obj #js {}]
                   (gobject/set prev fpart obj)
                   (recur (rest parts)
-                         obj))))))
-    (gobject/set js/window k f)))
+                         obj))))))))
 
 (def stns (sci/create-ns 'sci.script-tag nil))
+(def cljns (sci/create-ns 'clojure.core nil))
 
 (def namespaces
   {'sci.script-tag
-   {'defn (sci/copy-var defn stns)
-    '-export (sci/copy-var -export stns)}
-   'clojure.core {'println (sci/copy-var println stns)
-                  'prn     (sci/copy-var prn stns)}})
+   {'export (sci/copy-var export stns)}
+   'clojure.core
+   {'println     (sci/copy-var println cljns)
+    'prn         (sci/copy-var prn cljns)
+    'system-time (sci/copy-var system-time cljns)
+    'time        (sci/copy-var time cljns)}})
 
 (def ctx (atom (sci/init {:namespaces namespaces
                           :classes {'js js/window
@@ -54,8 +59,12 @@
   plug-in-name ;; unused for now
   (swap! ctx sci/merge-opts sci-opts))
 
+(def built-in (-> (eval-string "(map ns-name (all-ns))")
+                  set
+                  (disj 'user)))
+
 (c/defn- load-contents [script-tags]
-  (when-first [tag script-tags]
+  (if-let [tag (first script-tags)]
     (if-let [text (not-empty (gobject/get tag "textContent"))]
       (do (eval-string text)
           (load-contents (rest script-tags)))
@@ -67,7 +76,17 @@
                                     (let [response (gobject/get this "response")]
                                       (eval-string response))
                                     (load-contents (rest script-tags)))))]
-        (.send req)))))
+        (.send req)))
+    (eval-string (str/replace"
+(run! (fn [ns]
+        (let [nsn (ns-name ns)]
+          (when-not (contains? '%s nsn)
+            (run! (fn [var]
+                    (let [m (meta var)]
+                      (when (:export m)
+                        (sci.script-tag/export (str nsn  \".\" (:name m)) @var))))
+          (vals (ns-publics ns))))))
+  (all-ns))" "%s" built-in))))
 
 (js/document.addEventListener
  "DOMContentLoaded"