+
+\subsection{Parentheses}
+
+Parentheses are used for grouping of alternatives within the right-hand side
+of a production rule. Specifically, a right-hand side
+\begin{quote}
+ \syntax{$\alpha$ @($\beta_1$ | $\beta_2$ | $\cdots$ | $\beta_n$@) $\gamma$}
+\end{quote}
+where $\alpha$, $\beta_i$, and $\gamma$ are any sequence of nonterminal
+symbols or parenthesized groups, is equivalent to the right-hand side
+\begin{quote}
+ \syntax{$\alpha$ $b$ $\gamma$}
+\end{quote}
+together with the new production
+\begin{quote}
+ \syntax{$b$ ::= $\beta_1$ | $\beta_2$ | $\cdots$ | $\beta_n$}
+\end{quote}
+where $b$ is a new nonterminal symbol.
+
+Given the indexed-nonterminal notation described below, one might consider a
+group \syntax{@($\beta_1$ | $\beta_2$ | $\cdots$ | $\beta_n$@)} equivalent to
+\syntax{<group>@[$\beta_1$ | $\beta_2$ | $\cdots$ | $\beta_n$@]}, where
+\begin{quote}
+ \syntax{<group>@[$x$@] ::= $x$}
+\end{quote}
+
+
+\subsection{Indexed nonterminals}
+