diff -r 780c40aaad27 -r 349d706586ef cws/cw01.tex
--- a/cws/cw01.tex	Sat Dec 16 23:53:28 2017 +0000
+++ b/cws/cw01.tex	Mon Jan 15 23:15:34 2018 +0000
@@ -308,6 +308,9 @@
   \frac{price_{new} - price_{old}}{price_{old}}
   \]
 
+  If the change factor is defined, you should return it
+  as \texttt{Some(change factor)}; if not, you should return
+  \texttt{None}.
   
 \item[(2.b)] Write a function that calculates all change factors
   (deltas) for the prices we obtained under Task 1. For the running
@@ -320,7 +323,9 @@
 \end{verbatim}
 
   That means Google did a bit badly in 2010, while Apple did very well.
-  Both did OK in 2011.\\
+  Both did OK in 2011. Make sure you handle the cases where a company is
+  not listed in a year. In such cases the change factor should be \texttt{None}
+  (see 2.a).\\
   \mbox{}\hfill\mbox{[1 Mark]}
 
 \item[(3.a)] Write a function that calculates the ``yield'', or