Probably a cut-and-paste mistake.
\syntax{$x^*$ ::= <zero-or-more>$[x]$ ::=
$\epsilon$ @! <zero-or-more>$[x]$ $x$}
\end{quote}
\syntax{$x^*$ ::= <zero-or-more>$[x]$ ::=
$\epsilon$ @! <zero-or-more>$[x]$ $x$}
\end{quote}
-\item $x^+$ abbreviates @<one-or-more>$[x]$, denoting a sequence of zero or
+\item $x^+$ abbreviates @<one-or-more>$[x]$, denoting a sequence of one or
more occurrences of $x$:
\begin{quote}
\syntax{$x^+$ ::= <one-or-more>$[x]$ ::= <zero-or-more>$[x]$ $x$}
more occurrences of $x$:
\begin{quote}
\syntax{$x^+$ ::= <one-or-more>$[x]$ ::= <zero-or-more>$[x]$ $x$}