Added missing \end{datadesc}.
diff --git a/Doc/lib/libsys.tex b/Doc/lib/libsys.tex
index e16fbd5..2ea4c37 100644
--- a/Doc/lib/libsys.tex
+++ b/Doc/lib/libsys.tex
@@ -28,6 +28,7 @@
 may increase performance for programs using threads.  Setting it to a
 value <= 0 checks every virtual instruction, maximizing responsiveness
 as well as overhead.
+\end{datadesc}
 
 \begin{datadesc}{exc_type}
 \dataline{exc_value}