‘Hard to specify’

I read “two strings” literally. The strings are the only case-specific inputs. A fact read from a database, sensor, or anywhere else is another input, whether or not it appears in the function signature.

No function of those two strings can always provide such a ranking.

The bounty says the code must accept two strings as input. It doesn’t say only or at most or exactly two. But I see now that this may not have been clear – if you think it’ll help, I can edit the bounty to say ‘at least two’.

Also, when I wrote the bounty, I meant input as in ‘part of the function signature’. If the function body later reads from a database or prompts the user for input, that doesn’t in and of itself disqualify the submission.

… we cannot avoid guesswork about which changes preserve meaning …

If a good-faith submission prompted the user for answers about changes in meaning or other creative input (within reason), that also would not automatically disqualify the submission. (The proviso “within reason” matters or else bad actors will simply prompt the user to tell the program which explanation is harder to vary. As with all other bounties, any submission will have to meet not just the bounty terms but survive all criticism it may get.)

Here also, I can edit the bounty to clarify that user input at runtime is permissible.

I hope I’ve addressed all reasons the bounty might not be doable, but let me know if I missed something.