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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 think because your
titleCase("ABC DEF", "def")gives"Abc DEF"when I would expect it to give"Abc def"