diff --git a/lecture.cls b/lecture.cls
index 338f7b6..b486913 100644
--- a/lecture.cls
+++ b/lecture.cls
@@ -23,6 +23,7 @@
 \RequirePackage{totcount}
 \RequirePackage[hidelinks, unicode]{hyperref} %[unicode, hidelinks]{hyperref}
 \RequirePackage{bookmark}
+\RequirePackage{wasysym}
 
 \usetikzlibrary{quotes, angles}
 
@@ -142,3 +143,6 @@
 
 %josua
 \newcommand{\norm}[1]{\left\Vert#1\right\Vert}
+
+% contradiction
+\newcommand{\contr}{\text{\Large\lightning}}