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Thank you! Yes, it works:)

Thanks a lot for the response! I tried to replace d[player].add(group) with d[player].update(group) as you suggested in the other comment, but it still does not work for this input.

Correct! The kata's test cases should be updated, if possible. Thank you for the comment. It could help people who come across this solution.

Edit: The error is probably in the second last line -- see the other comment below.

Please check this input:
s = [
["AB", "AC","DE"],
["AD", "AE", "BC"]]

The function returns true, though it should be false, since A is playing twice each day. Thank you!