This is not quite fool-proof. I wanted something that was a proof-of-concept but does not do complete error checking so ... (for example 12 + 3 + does not do what you would expect which is to give a result of 15) Type in your numbers and choose one of the two functions (addition or subtraction). Use Clear to start over.
Credit goes to one of my students who gave me the idea. He wanted to create a calculator and I had to go home and create one so I knew how to help him with his! It took about 3 hours. I used lists in order to keep track of the digits.
