--- a/hws/hw05.tex	Mon Oct 21 15:02:54 2013 +0100
+++ b/hws/hw05.tex	Tue Oct 22 23:17:53 2013 +0100
@@ -105,9 +105,17 @@
 this automaton. (Hint: If you use Brzozwski's method, you can assume
 Arden's lemma which states that an equation of the form $q = q\cdot r + s$
 has the unique solution $q = s \cdot r^*$.)\
+
+\item Recall the definitions for $Der$ and $der$ from the lectures. 
+Prove by induction on $r$ the property that 
+
+\[
+L(der\,c\,r) = Der\,c\,(L(r))
+\]
+
+holds.
 \end{enumerate}
 
-
 \end{document}
 
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