I'm guessing that you're probably using a set without any import, which is quite limited.
What yo're looking for are hash sets, available after you import them. Do `import sets` and then use [https://nim-lang.github.io/Nim/sets.html](https://nim-lang.github.io/Nim/sets.html) for a guidance how to do stuff you're interested in. (E.g. in python you would use `add`, here is `incl`)
