It should be clearly stated in the description the input should not be modified. From the current description, several users interpreted that an inline modification of the list was required, or at least possible.

great, just solved it, thanks for the motivational speech ;)

Really cool kata, it should be much more popular. The description seems scary, but the task itself is not that hard. Give it a try!

Do not modify the nested list in your implementation or you won't pass the test ;)

for 1, raising an error is part of the criteria, but I agree with everything else

clever move !
great work

i just now tried the same code and works fine for me

It took me some time to understand that given nested list should not be modified in the process. I think a clear mentioning of such requirement should be provided in the task section.

This is no longer needed for python 3

Maybe just start with simpler kata?
Not an issue, in any case.

cheers