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 modality, 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 you would type 1 + 2. Using RPN, you would type 1 2 +. The operator comes after the operands.

Using an algebraic calculator, you would need to press an = key to indicate that you want a result. With an RPN calculator this is unnecessary because the operator processes immediately, serving a double purpose. On the other hand, an RPN calculator often requires an Enter keypress to push an operand onto the stack.

We can easily implement...