Dispatch
 Usage
• Dispatch[  -> ,  -> , ...  ]用来生成一个法则列表的一个最优化的分派表表示.由 Dispatch产生的这个对象可以用来给出 expr /. rules中的法则.
Notes
• Dispatch的使用绝不会影响所得到的结果，但可以使长的法则列表应用更快. • 当你对一个象 expr /. rules的表达式赋值时，法则列表通常按顺序被检索. 诸如 a[1]->a1和 a[2]->a2的法则不能同时使用，从而不必都明确被检索.Dispatch 生成一个分派表，对于特定的输入表达式，它使用哈希码来指明法则的哪些集合不必真正被检索. • 使用= 和:= 进行赋值所产生的法则列表在适当的时候会自动使用分派表来优化.
 Further Examples
Here is a list of 1000 rules.
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This sets up a dispatch table that make these rules faster to use.
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You can apply either ordinaryrules or dispatchrules using /., the slash-dot operator (ReplaceAll).
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The dispatch rules go through all the rules significantly faster. (The results depend on the platform that you are using.)
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Now we make the table of rules four times as long and we again set up the corresponding dispatch table.
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While the time to do replacements with ordinaryrules increases, the time using dispatchrules is not much more than before.
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We clear the definitions that we made.
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