check

book.tex

@@ -4302,11 +4302,11 @@ grammar rule that was applied, which in this example is \code{'exp'}
 for all three \code{Tree} nodes. Each tree object also has a
 \code{children} field that is a list containing trees and/or tokens.
 
-A grammar is ambiguous when there are strings that can be parsed in
-more than one way. For example, consider the string \code{'1+2+3'}.
-Using the grammar comprised of rules \eqref{eq:parse-int} and
-\eqref{eq:parse-plus}, this string can parsed in two different ways,
-resulting in the two parse trees shown in
+A grammar is \emph{ambiguous}\index{subject}{ambiguous} when there are
+strings that can be parsed in more than one way. For example, consider
+the string \code{'1+2+3'}.  Using the grammar comprised of rules
+\eqref{eq:parse-int} and \eqref{eq:parse-plus}, this string can parsed
+in two different ways, resulting in the two parse trees shown in
 figure~\ref{fig:ambig-parse-tree}.
 
 \begin{figure}[tbp]