Observation (3) is actually not an issue. I got confused. I was talking about situation when player doesn't play every day. It is checked by test case 'can detect an unknown player', on which i stumbled. Yesterday for some reason it seemed that this case is not specified in instructions. But actually it is clearly specified in '(1) that each golfer plays exactly once every day' (c). Sorry.

About the case of empty list. Adding test case indeed is not necesary. Addressing that case in instructions is enough. Which you did. Thank you.

Actually by adding a test case that N=0 golfers should be handled, it broke every single one of the nine solutions people submitted, so I think I should go back to not handling that at all.

Wow, thank you so much for your feedback. I have corrected the test case typos, and clarified the behavior for emptiness and even added a visibile test case for it.

As for observation (3) are you suggestion the problem description calls out a requirement that the sizes of the groups must be the same every day?