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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.
No random tests.
Step 1: default string, or really target string? "target string" sounds like it's actually an argument, and it then might have a different length.
Locking correct characters contradicts step 3.
It would probably be good to make the description internally consistent, and not make any promises that you're going to break later.
How do you define "closest"?