Stack
 Usage
• Stack[ ] 显示当前计算堆栈,并返回与当前正进行的计算相联系的标记列表.
• Stack[pattern] 返回当前正进行计算并与指定模式匹配的表达式列表.
Notes
• Stack[_] 显示当前正被计算的表达式. • 你可以在一个对话的内部调用Stack来查看对话进行的方式. • 由Stack[pattern]返回的列表中, 每个表达式都被HoldForm包装. • Stack[ ] 的最大长度受$RecursionLimit的限制. • Stack 具有属性 HoldFirst.
 Further Examples
Stack[_] gives the expressions that are being evaluated at the time when it is called, in this case including the Print function.
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 Stack gives the tags associated with the evaluations that are being done when it is called.
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 Here is the standard recursive definition of the factorial function.
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This evaluates fac[10], starting a dialog when it encounters fac[4].
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This shows what objects were being evaluated when the dialog was started.
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This ends the dialog.
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StackInhibit prevents Print from being included on the stack.
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 StackBegin[expr] uses a fresh stack in the evaluation of expr.
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