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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 instruction is a bit misleading. Return nil if more there than 10, The correct one is return nil if more than nine
The instruction is unclear. I thought that the block will only need an argument and I need to call it once for every element in the list. But actually it needs two argument and it needs to be call for every adjacent pair on the list.