Contributing to solutions? (searched, but old links are broken)

I’d like to contribute a set-based solution to this page, but can’t figure out where to go. Searching the forum, I found this link, but the only answer as far as the “guide” folder 404s. I’ve tried grepping through the monorepo, but haven’t been able to find the right file there, either.

So, where does the source for this hint/solution page live?