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of variables and/or terminals; rather than using ordered pair notation, production rules are usually written using an arrow operator with as its left hand side and as its right hand side:.

It is allowed for to be the empty string, and in this case it is customary to denote it by . The form is called an ?-production.

It is common to list all right-hand sides for the same left-hand side on the same line, using | (the vertical bar)

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