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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.
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That's because negative numbers would be out of scope. As the description says: "A string representation of an integer will contain no characters besides the ten numerals "0" to "9"." A negative number would also have a "-".