It has been a LONG time since I did any real math and never took statistics, but wouldn't x^(-1) just be 1/(x)? I don't know if that equates to "everything that isn't x". I feel like there's a specific way to write that, but a negative exponent is not that, I don't think, but also I have no idea.

I looked it up. Looks like this stuff is maybe from set theory? Which I sooorrrrt of remember doing at some point?

My best guess is your professor either said something from this, or you misremembered, or I'm totally off base and I'm still curious.

https://en.wikipedia.org/wiki/Complement_(set_theory)