Trying to remember where I saw an article about redesigning calculator user interfaces... Parts I recall: Removing the '=' key and moving to an RPN-ish stack. Using '+', '-', 'x' and a kind of reverse-multiply as single-press buttons (all for integer based maths). Another set of four keys provided more complicated functionality. Using little triangles at the top to indicate whether the number is negative, or at the bottom for a decimal point. Has anyone else seen this?