How do you rank the difficulty of Koans? My intuition does a very good job by now, but caching it out has always resulted in obviously wrong corner cases.

The method I used is very close to this one, but what you say is true. There are edge-cases that aren't quite right. For instance, the most difficult "medium" Koans seem more difficult than the least difficult "hard" Koans.

"A Koan has the Buddha-nature if and only if all the pieces are ungrounded, except for blue pieces." is unclear to me. I am not sure whether blue pieces must be grounded or may be grounded.

Would "A Koan has the Buddha-nature if and only if all the non-blue pieces are ungrounded, and all the blue pieces are grounded" be less ambiguous?

Nice job!

Thanks.