Ask HN: Exercise that makes source-to-source compiler wheel

Sorry if this is an inappropriate post/against the rules. The curiosity is killing me.

What's the name for the exercise of making a source-to-source compiler wheel? As in you have source in language X that compiles to lang Y, which compiles to lang Z, and back again to X?

I saw one on HN a while ago for 20+ languages and I can't find it, searched everywhere.