--- a/slides/slides04.tex	Wed Oct 18 08:36:44 2017 +0100
+++ b/slides/slides04.tex	Wed Oct 18 11:02:44 2017 +0100
@@ -889,7 +889,7 @@
 \frametitle{Simplification}
 
 \begin{itemize}
-\item If we simplify after the derivative, then we are builing the
+\item If we simplify after the derivative, then we are building the
 value for the simplified regular expression, but \emph{not} for the original
 regular expression.
 \end{itemize}