`in` is operator in Nim which uses proc `contains` If you want to use the `in` operator, you should provide binary proc `contains` .
Since `contains` is widely implemented in various stdlib (it's needed to if we want to use `in` ), you can check from [theindex](https://nim-lang.org/docs/theindex.html#contains) which one you need. I suggest you check the contains proc from system module.
