Copies the current topmost value on the evaluation stack, and then pushes the copy onto the evaluation stack.
复制计算堆栈上当前最顶端的值，然后将副本推送到计算堆栈上。
Done using an arithmetic expression evaluation stack: file input from the keyboard or an arithmetic expression, calculate and display the results.
利用栈完成算术表达式求值：从键盘或文件中输入算术表达式，计算其结果并显示。
Done using an arithmetic expression evaluation stack: file input from the keyboard or an arithmetic expression, calculate and display the results.
利用栈完成算术表达式求值：从键盘或文件中输入算术表达式，计算其结果并显示。
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