diff -r 1af5ec39d006 -r a6aa52ecc1c5 slides/slides01.tex
--- a/slides/slides01.tex	Thu Sep 28 19:07:37 2017 +0100
+++ b/slides/slides01.tex	Tue Oct 03 23:01:06 2017 +0100
@@ -529,7 +529,7 @@
 
 \begin{textblock}{6}(2,7.5)
   \begin{tabular}{@ {}rrl@ {\hspace{13mm}}l}
-  \bl{$r$} & \bl{$::=$}  & \bl{$\ZERO$}  & null\\
+  \bl{$r$} & \bl{$::=$}  & \bl{$\ZERO$}  & nothing\\
          & \bl{$\mid$} & \bl{$\ONE$}       & empty string / \pcode{""} / $[]$\\
          & \bl{$\mid$} & \bl{$c$}                         & character\\
          & \bl{$\mid$} & \bl{$r_1 + r_2$}  & alternative / choice\\