diff --git a/11_extensibility.scm b/11_extensibility.scm
new file mode 100644
index 0000000..48f29ba
--- /dev/null
+++ b/11_extensibility.scm
@@ -0,0 +1,40 @@
+;; -*- geiser-scheme-implementation: guile -*-
+
+(if (our-test)
+    (begin
+      (do-thing-1)
+      (do-thing-2)))
+
+(define (when test . body)
+  `(if ,test
+       ,(cons 'begin body)))
+
+(when '(our-test)
+  '(do-thing-1)
+  '(do-thing-2))
+                                        ;  (if (our-test) (begin (do-thing-1) (do-thing-2)))
+
+(define-macro (when test . body)
+  `(if ,test
+       ,(cons 'begin body)))
+
+(when (our-test)
+  (do-thing-1)
+  (do-thing-2))
+
+(define-syntax-rule (when test body ...)
+  (if test
+      (begin body ...)))
+
+(define-syntax-rule (for (item lst) body ...)
+  (for-each (lambda (item)
+              body ...)
+            lst))
+
+(for (str '("strawberries" "bananas" "grapes"))
+     (display
+      (string-append "I just love "
+                     (string-upcase str)
+                     "!!!\n")))
+
+;  #<unspecified>