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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.
I believe many have written this before, however, I believe what's written it's not very clear.
I could only truly understand what was being asked when I looked at the test cases and comparing them.
There should be something closer to a sorted string of individual letters with the combined strings. Too simple like this, probably.