Reverse Polish Notation is the notation used on Hewlett Packard and other high end calculators. The notation is elegant, allowing unlimited expression complexity without requiring the use of parentheses.
RPN is based on the use of a stack data structure. In CCICAP, the stack has three storage positions. The three positions can be labeled x, y, and z. Whenever a new value is entered into a calculation, it is placed in position x while the value that was in position x is moved to y, the value that was in position y is moved to z, and the value that was in position z is lost.
Operations involving the values stored in the stack positions are of three types: binary, unary, and stack. The values must be stored in the stack before the operation is executed.