-Fortunately, Sod is syntactically quite simple. I've used a little slightly
-unusual notation in order to make the presentation easier to read. For any
-nonterminal $x$:
+Fortunately, Sod is syntactically quite simple. The notation is slightly
+unusual in order to make the presentation shorter and easier to read.
+
+Anywhere a simple nonterminal name $x$ may appear in the grammar, an
+\emph{indexed} nonterminal $x[a_1, \ldots, a_n]$ may also appear. On the
+left-hand side of a production rule, the indices $a_1$, \ldots, $a_n$ are
+variables which vary over all nonterminal and terminal symbols, and the
+variables may also appear on the right-hand side in place of a nonterminal.
+Such a rule stands for a family of rules, in each variable is replaced by
+each possible simple nonterminal or terminal symbol.
+
+The letter $\epsilon$ denotes the empty nonterminal
+\begin{quote}
+ \syntax{$\epsilon$ ::=}
+\end{quote}
+
+The following indexed productions are used throughout the grammar, some often
+enough that they deserve special notation.