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.