NaturalNumberCalculator
/**
* Natural Number Calculator application.
*
* This is a very simple “RPN” (Reverse Polish Notation) calculator.There are
* two operands, both of which always have natural number values.Direct entry
* of a number is always to the bottom operand in the display. The”Clear”
* button sets the bottom operand to 0. The “Swap” button exchangesthe values
* of the two operands. The “Enter” button copies the value of thebottom
* operand to the top operand. Each operator button operates on thetwo operands
* in their natural order as displayed in the interface (e.g., “-“subtracts the
* bottom operand from the top operand), and each replaces thebottom operand
* with
OR
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