I was reading this answer about Turing machines and it refers to a bussiness-card-sized one, which claims to be a universal Turing machine, based on this paper. However, I don't understand the logic of that particular machine. I understand that in a universal Turing machine the "program" is just another machine encoded in the tape of the universal Turing machine. But, how is it encoded in this case? How would you write a program that adds two numbers for this machine, for instance?