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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.
That's why they use the term interval, not a set, because it is not a set of integers (for which we can calculate a number of elements) but an interval of real numbers on the number line (which has a length). It get clearer when you get to the examples: interval [1, 5] has a length of 4 and a set {1, 2, 3, 4, 5} would have 5 elements.