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.
+Such a rule stands for a family of rules, in which each variable is replaced
+by each possible simple nonterminal or terminal symbol.
The letter $\epsilon$ denotes the empty nonterminal
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