Each expression class has an evaluate method, and an enumerate method.
每个表达式类都有一个evaluate方法和一个enumerate方法。
Each expression class has a simplify method, to allow expression rewriting.
每个表达式类都有一个simplify方法，以允许表达式重写。
Each expression class has a simplify method, to allow expression rewriting.
每个表达式类都有一个simplify方法，以允许表达式重写。
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