diff --git a/lectures/L21-slides.tex b/lectures/L21-slides.tex
index bb15a76..3816695 100644
--- a/lectures/L21-slides.tex
+++ b/lectures/L21-slides.tex
@@ -431,7 +431,7 @@
 \end{lstlisting}
 
 
-The following functions are used to swap two values, only if the old value matches the expected (i.e., what was provided as the second argument):
+The following functions are used to swap two values, that is, if the current value of *ptr is oldval, then write newval into *ptr:
 
 \begin{lstlisting}[language=C]
 bool __sync_bool_compare_and_swap( type *ptr, type oldval, type newval );