NHoldAll
 Usage
• NHoldAll 是一个属性值，用来指定一个函数的所有参数中没有会被N影响的参数.
Notes
• NHoldAll, NHoldFirst和NHoldRest在保证函数的参数为准确整数，而不被N转换为近似数时有用.
 Further Examples
Usually N goes inside functions and gets applied to each of their arguments.
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This tells Mathematica not to apply N to any of the arguments of f.
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Now both of the arguments of f are left in their exact form.
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