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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.
The task sounds as 'if given object is a digit (0-9)'. 'is a single digit' would make it more clear. Because 0-9 is a typical pattern for regex-related thoughts and it really misleads.