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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.
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Indeed, it does call triangular(0), which returns 0, which is added to the recursive summation, causing no change to the value:
triangular(4) =
triangular(3) + 4 =
triangular(2) + 3 + 4 =
triangular(1) + 2 + 3 + 4 =
triangular(0) + 1 + 2 + 3 + 4 =
0 + 1 + 2 + 3 + 4 =
10