diff -r d41956ea544e -r 82a1315c128d slides/slides01.tex
--- a/slides/slides01.tex	Mon Sep 21 13:16:02 2020 +0100
+++ b/slides/slides01.tex	Wed Sep 23 11:34:43 2020 +0100
@@ -1056,6 +1056,32 @@
 \end{frame}
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%   
 
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+\begin{frame}[c]
+\frametitle{The Meaning of a Regex}
+
+ ...all the strings a regular expression can match.   
+
+\begin{center}
+ \begin{tabular}{rcl}
+ \bl{$L(\ZERO)$}  & \bl{$\dn$} & \bl{$\{\}$}\\
+ \bl{$L(\ONE)$}     & \bl{$\dn$} & \bl{$\{[]\}$}\\
+ \bl{$L(c)$}            & \bl{$\dn$} & \bl{$\{[c]\}$}\\
+ \bl{$L(r_1 + r_2)$}    & \bl{$\dn$} & \bl{$L(r_1) \cup L(r_2)$}\\
+ \bl{$L(r_1 \cdot r_2)$} & \bl{$\dn$} & \bl{$L(r_1) \,@\, L(r_2)$}\\
+ \bl{$L(r^*)$}           & \bl{$\dn$} & \\
+  \end{tabular}
+\end{center}
+
+\begin{textblock}{14}(1.5,13.5)\small
+\bl{$L$} is a function from regular expressions to 
+sets of strings (languages):\smallskip\\
+\bl{\quad$L$ : Rexp $\Rightarrow$ Set$[$String$]$}
+\end{textblock}
+
+\end{frame}
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+
 
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 \begin{frame}[c]