fix issue #70

book.tex

@@ -10350,12 +10350,12 @@ arrays by laying out each row in the array, one after the next.
 
 In this chapter we study the implementation of mutable tuples, called
 vectors in Racket. This language feature is the first to use the
-computer's \emph{heap}\index{subject}{heap} because the lifetime of a Racket
-tuple is indefinite, that is, a tuple lives forever from the
+computer's \emph{heap}\index{subject}{heap} because the lifetime of a
+Racket tuple is indefinite, that is, a tuple lives forever from the
 programmer's viewpoint. Of course, from an implementer's viewpoint, it
 is important to reclaim the space associated with a tuple when it is
-no longer needed, which is why we also study \emph{garbage collection}
-\emph{garbage collection} techniques in this chapter.
+no longer needed, which is why we also study \emph{garbage
+collection} \index{garbage collection} techniques in this chapter.
 
 Section~\ref{sec:r3} introduces the \LangVec{} language including its
 interpreter and type checker. The \LangVec{} language extends the \LangIf{}