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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.
They could also pass strings, booleans, bananas and unicorns to that function. In any case, the description says that it is guaranteed that the input is correct and has only valid values.
But we are not told in the description that such values may be passed, only that 0-9 appear in the solutions to validate.