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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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Sometimes creating a list for the purpose of calling sum is marginally faster (especially if the list gets longer)
Hence I upvoted this answer along with a very similar one that uses map
I believe this solution is incorrect:
for example digital_root(91) will return 10 while it should return 1