In reply to @aryzen "how do I look": Generally when you have only keywords you
can look through the nim manual.
It's as close to a complete spec of the language that explains what it does as
you're going to get
https://nim-lang.org/docs/manual.html

I have it bookmarked for that very reason.

For the when statement specifically
https://nim-lang.org/docs/manual.html#statements-and-expressions-when-statement
https://nim-lang.org/docs/manual.html#statements-and-expressions-when-expression