2013-03-26: added a let test.

added a let test.

diff --git a/example.w b/example.w
--- a/example.w
+++ b/example.w
@@ -41,4 +41,8 @@ defun _ : \:
 __ . \:
 
 \_ b
-      
\ No newline at end of file
+
+defun d : 
+      let 
+          : a b
+            c d