Flat
 Usage
• Flat 是一个属性，它可以赋给一个符号f , 指定涉及嵌套函数 f 的所有表达式应当被压平。这个属性在模式匹配中说明。
Notes
• Flat 相应于数学上的结合律。 • 对一个有属性 Flat的符号 f , f[f[a, b], f[c]]自动减小到 f[a, b, c]. • 象Plus, Times 和 Dot的函数是 Flat. • 对一个 Flat 函数，在模式 f[x_, y_]中变量x和y相应于任何参数序列。 • Flat 属性必须在对Flat函数定义任何值被分配。
 Further Examples
Clear all of the rules and attributes for a symbol fff.
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Set the Flat attribute for fff.
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Inputs with a head of fff will now be flattened automatically over fff.
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The Flat attribute also has an effect in pattern matching. This rule applies because of the Flat attribute.
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Remove the Flat attribute from fff.
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Without the Flat attribute, this rule is not used.
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Associative operations, such as Plus, have the Flat attribute.
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This rule is applied because of the Flat attribute of Plus.
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This also works.
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