--- a/hws/hw05.tex	Thu Sep 19 16:32:26 2024 +0100
+++ b/hws/hw05.tex	Thu Sep 19 19:21:52 2024 +0100
@@ -56,6 +56,22 @@
 %where the three dots stand for arbitrary characters, but not
 %comment delimiters.
 
+\item The \emph{not}-regular expression is definitely useful for recognising
+  comments for example, but can
+  sometimes be quite unintuitive when it comes to deciding which strings are
+  matched or not. Consider
+
+  \[
+  (\sim{}a)^*  \quad\text{and}\quad \sim{}(a^*)\;.  
+  \]  
+
+  What is the language of each regular expression?
+
+  \solution{
+    The first one is ``all strings, except $[a]$''; the second
+    ``all strings except strings of the form $a^*$''.
+  }
+  
 \item Define the following regular expressions 
 
 \begin{center}