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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, that was irritating. ;)
Have you tried printing the input? I think you're dealing with a special arithmetic sequence.
In the PHP version, I'm failing the random tests - it seems some generated sequences do not have a number missing from the sequence?