符号 $3的位置是通过 @3 进行引用的。
yacc允许您通过符号名引用表达式的组成部分。
Yacc allows you to refer to the components of an expression by symbolic names.
add函数的第一个参数是一个符号名，可以在notebook中通过它引用生成的页：下面我们将使用它。
The first argument to the add function is a symbolic name, by which the generated page can be referred to in the notebook: we will make use of it below.
add函数的第一个参数是一个符号名，可以在notebook中通过它引用生成的页：下面我们将使用它。
The first argument to the add function is a symbolic name, by which the generated page can be referred to in the notebook: we will make use of it below.
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