I am a non-mathematician. I'm reading up on set theory. It's fascinating, but I wonder if its found any 'real-world' applications yet. For instance, in high school and we were learning the properties of i, a lot of the kids wondered what it was used for. The teacher responded that it was used to describe the properties of electricity in circuits. So is there a similar practical app of set theory? Something we wouldn't be able to do or build without set theory?