added hint about priority queue

book.tex

@@ -2699,7 +2699,7 @@ conclusion:
 	retq
 \end{lstlisting}
 \end{minipage}
-\caption{Example with function calls.}
+\caption{An example with function calls.}
   \label{fig:example-calling-conventions}
 \end{figure}
 
@@ -3380,7 +3380,11 @@ shown in Figure~\ref{fig:reg-alloc-passes}.
   mapping of variables to their colors (represented as natural
   numbers). By creating this helper function, you will be able to
   reuse it in Chapter~\ref{ch:functions} when you add support for
-  functions.
+  functions. The support code includes an implementation of the
+  priority queue data structure in the file
+  \code{priority\_queue.rkt}, which might come in handy for
+  prioritizing highly saturated nodes inside your \code{color-graph}
+  function.
 
   Once you have obtained the coloring from \code{color-graph}, you can
   assign the variables to registers or stack locations and then reuse