explicate_control fix

book.tex

@@ -3469,7 +3469,7 @@ that we have alluded to earlier. Here is the definition.
 \end{enumerate}
 \end{definition}
 
-We recommend implementing \code{explicate\_control} using two mutually
+We recommend implementing \code{explicate\_control} using two
 recursive functions, \code{explicate\_tail} and
 \code{explicate\_assign}, as suggested in the skeleton code in
 Figure~\ref{fig:explicate-control-Lvar}.  The \code{explicate\_tail}