great

There is no ISO C 2020 standard. There is no reason to assume this valid syntax will be invalidated in the future.

No

Is this valid ISO 2020 syntax in C?

I've never seen this syntax before and tried googling it with no success. Can someone briefly explain it to me??

Funny how I came up with almost the same strings.

"Out of interest do you know if this kind of data structure has been used anywhere?"

I second that! Searched around the web for clues but found none so far.
Also wonder what's the optimal data structure for implementing it - some kind of DAG? I'd very much like to see a purely functional solution.

Approved

Very very easy compared to other C kata(s?) at 6 kyu. Fun though :)

Same problem here, passes example tests fine, then after submitting there are a load of errors with the final tests.

Yea I did find the error messages a bit naff, but I couldn't find any docs for php test cases, so ended up just copying and pasting from another kata, then adapting them.

I've updated the test cases based on the details you've provided, so errors should be more descriptive now.