There is an operator | for alternatives in a grammar rule.
For example:
S: A | B;
terminals
A: 'a';
B: 'b';
This grammar accepts two strings: a and b.
The grammar rule S: A | B; means S can be parsed by parsing A or by parsing B.
Each operand of alternatives is called a production.
In the grammar rule S: A | B;, there are two productions.
One is A and the other is B.
For a grammar rule that has no operators of alternatives, the grammar rule has only one production.
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