Saving and Reusing Numbers
Sometimes you might want to include the result of a previous calculation in a new calculation. There are several ways to reuse numbers.
The History Stack of Numbers
When you start a new operation, the previous result moves out of the display but is still accessible. Up to four lines of numbers are saved: one in the display and three hidden. These lines make up the history stack.
"Invisible" numbers 
remaining from previous results.
The ], [, and @~keys “roll” the history stack down or up one line, bringing the hidden results back into the display. If you hold down [ or ], the history stack wraps around on itself. However, you cannot roll the history stack when an incomplete calculation is in the display. Also, you cannot gain access to the stack while using lists (SUM, CFLO) in ALG mode, or SOLVE in either ALG or RPN mode. All numbers in the history stack are retained when you switch menus.
Pressing @xexchanges the contents of the bottom two lines of the display.
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