--- a/slides04.tex	Wed Oct 17 08:51:13 2012 +0100
+++ b/slides04.tex	Wed Oct 17 23:22:43 2012 +0100
@@ -515,7 +515,7 @@
 \begin{center}
 \bl{($\delta$(q,c), $\delta$(p,c))}
 \end{center} 
-are marked. If yes, then also mark bl{(q, p)} 
+are marked. If yes, then also mark \bl{(q, p)} 
 \item Repeat last step until no chance.
 \item All unmarked pairs can be merged.
 \end{enumerate}