diff -r bdd12391d345 -r 34f77b976b88 hws/hw04.tex
--- a/hws/hw04.tex	Tue Sep 29 19:35:11 2020 +0100
+++ b/hws/hw04.tex	Tue Sep 29 21:52:52 2020 +0100
@@ -41,7 +41,13 @@
   In case they can, can you give the corresponding token
   sequences.
 
-\item Assume that $s^{-1}$ stands for the operation of reversing a
+\item Assume $r$ is nullable. Show that
+  \[ 1 + r + r\cdot r \;\equiv\; r\cdot r
+  \]
+
+  holds.
+
+\item \textbf{(Deleted)} Assume that $s^{-1}$ stands for the operation of reversing a
   string $s$. Given the following \emph{reversing} function on regular
   expressions