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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.
I personnaly prefer map/reduce because its how I learned the concept, from big data map/reduce
In my experience, it's much more common to see people use map.
In ruby, map and collect are synonymous, which one you use is based purely on preference.