update lecture cls

lecture.cls

@@ -105,3 +105,9 @@
 
 % activate my colour style
 \lstset{style=mystyle}
+
+% better stackrel
+\let\oldstackrel\stackrel
+\renewcommand{\stackrel}[2]{%
+    \oldstackrel{\mathclap{#1}}{#2}
+}%