A Simple RPN Calculator with Deque

An RPN (Reverse Polish Notation) calculator is a stack-based calculator that uses postfix notation, where the operator follows the operands. It's commonly used in printing calculators and, notably, the HP 12C, the most popular electronic calculator of all time.

After becoming familiar with its operational modalityThis refers to the specific mode or method by which a system, process, or organization operates or functions to achieve its goals. many people prefer an RPN calculator. (I've been using the HP 12C and 16C since they were first introduced in the early 1980s.) For example, using conventional algebraic notation, to add $1$ and $2$ we would type $1 + 2$. Using RPN, we would type $1\:2\: +$ ...