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.
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.