The functions setmetatable() and getmetatable() modify and query an object's metatable, respectively.

函数 setmetatable() 和 getmetatable() 分别对对象的元表进行修改和查询。

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The keys of the metatable are called events; the values (in other words, the functions) are called metamethods.

元表的键值称为事件；值（换而言之就是函数）称为元方法。

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The function String() takes a string, string, wraps it in a table ({value = s or ''}), and assigns the metatable mt to the table.

函数 String() 接收一个字符串 string，将其封装到一个表（{value = s or ''}）中，并将元表 mt 赋值给这个表。

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