fixes #814 (#1015)

Author: martin-henz

xml/chapter4/section2/subsection1.xml

@@ -342,6 +342,11 @@ factorial(5);
       <JAVASCRIPT>functions</JAVASCRIPT>
     </SPLITINLINE>
     work in a normal-order language?
+    <SOLUTION>
+      (provided by GitHub user joeng03)
+When evaluated in default (‘strict’) mode, the code undergoes applicative order reduction and runs into an infinite loop because it needs to evaluate the arguments of the unless function, but the factorial function does not have a terminating condition.
+When evaluated in lazy mode, the code undergoes normal order reduction. It produces the correct output because the unless function is applied before its arguments are evaluated.      
+    </SOLUTION>
   </EXERCISE>
 
   <EXERCISE>