Yacc can recognize patterns of tokens; for instance, as in the example above, it can recognize that an expression can consist of a value, either a plus or minus sign, and another value.
yacc可以识别出记号的模式；例如，如上面例子中所示，它可以识别出一个表达式可能由一个值、一个加号或者减号以及另一个值构成。
This means that an expression can take any of several forms; for instance, a variable, a plus sign, and a number could be an expression.
这意味着表达式可以是几种格式中的任意一种；例如，一个变量、一个加号或者一个数字都可以是一个表达式。
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