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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.
Maybe adding an example of a diagonal island would be a good idea to make it clearer.
Have you read the instructions? That same example is there and it says they're 2 islands.