very interesting, the idea of reducing for the accumulator, but not caring about the return value. I think you might be able to make a use case for how this is idomatic to the specification, but I marked clever, because I think the meaning is a little lost in the code.

woah, jason!! You finished multiple days early?

this is very close to my solution. I will add that the conditional add remove is exactly what the #^ method does, so that would make the code a little cleaner

what does set.add return if it exists/doesnt exist?