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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.
The usage of "same" in the description can be a bit tricky to parse, but a look at the test cases sort of helps clarify things.
The description says, "...all of them (the other elements) are the same, except for one single number."
In your example case, although 2 is the single number, 1 and 3 are not all the same. Therefore, it would not be what the kata is testing for.
In other words, the kata is looking for examples that take on the template of: [x, x, x, x, y, x, x] or [y, x, x, x, x] but not [x, y, y, z, z, z, z, z, z].
You'll be wrong with this case.
stray([1,1,1,2,3,3,3])
It returns 0.