Simple fixed-point decimal arithmetic, Reverse Polish Notation calculator. RPN
calculators may be in the minority but they are easy to learn and very
efficient. The idea is that the operator follows the operands, so that instead
of typing 2 + 2 =, one types 2 enter 2 enter +. The left arrow either deletes
the last digit or drops the last item from the stack. The up arrow show a few
other functions such as setting the number of decimal points, swapping items on
the stack and the use of exponents.