4. The calculation device of claim 1 , wherein, in response to each new press of the shortcut key in rapid succession, the hardware processor cycles through the list of the previously entered variables.
4. The calculation device of claim 1 , wherein, in response to each new press of the shortcut key in rapid succession, the hardware processor cycles through the list of the previously entered variables. 5. The calculation device of claim 4 , wherein, if only one variable has been entered in the mathematical expression, then the hardware processor retrieves the one variable from the memory and outputs the one variable to the display as part of the mathematical expression.