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Collections are a way for you to organize kata so that you can create your own training routines. Every collection you create is public and automatically sharable with other warriors. After you have added a few kata to a collection you and others can train on the kata contained within the collection.
Get started now by creating a new collection.
Yes, you're right, it's a classic off-by-one error.
The testcases are not picking it up, though.
I believe this solution returns the wrong result:
items = ['a','b','c']
solution(items,3,'d')
=> nil
but should be 'd'