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依据SysY文档，语义约束如下：
- [ ] CompUnit:
1. [ ] 存在且仅存在一个标识为'main'，无参数，返回类型为int的FuncDef(函数定义)。
2. [ ] CompUnit中的顶层变量/常量Decl、函数定义FuncDef都不可以重复定义同名的Ident，即便标识符的类型不同。
- [ ] ConstDef:
- [ ]
