This is a calculator which uses Reverse Polish Notation, or RPN. A full write-up in Japanese (Google Translate link) carries an impressive level of detail about the project, but in short ...

Why reach for a bland, commercially available calculator when you be using a model that employs RPN (Reverse Polish Notation) in its calculations and be a custom build all at the same time?

For this, we use an RPN or a Risk Priority Number. Risk Priority Number (RPN) is an integral part of the creation of a ...