what this is

An honest calculator. No equals key, no operator precedence, no parentheses — just a stack. You push numbers, then apply an operation to the top of the stack. It is the opposite of what phone calculators teach, and it never lies about what it did.

• 2 enter 3 × — push 2, push 3, multiply → 6
• 5 enter 3 − — push 5, push 3, subtract → 2 (5 − 3, always)
• ENTER — pushes the number you typed; with no entry, it copies the top
• swap — exchange the top two values (x↔y)

RPN was born in 1954 with Jan Łukasiewicz's prefix notation, made famous by HP calculators from 1968 on. Engineers swear by it: one pass, no ambiguity, every step visible on the stack.