diff --git a/inputs/lecture_04.tex b/inputs/lecture_04.tex
index 90ea9e6..20bd08d 100644
--- a/inputs/lecture_04.tex
+++ b/inputs/lecture_04.tex
@@ -5,7 +5,7 @@
 \section{$\ZFC$}
 
 % 1900, Russel's paradox
-Russel's Paradox:
+\todo{Russel's Paradox}
 $\ZFC$ stands for
 \begin{itemize}
     \item \textsc{Zermelo}’s axioms (1905), % crises around 19000