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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.
A little nitpicky, but you only need to run &block on each element once, not twice: find list.select(&block) as var, and then return as the second element all elements in list and not in var. If &block is expensive, it might be desirable not to run it twice.
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I'm not sure a Set is guaranteed to retain order, see http://www.ruby-doc.org/stdlib-1.9.3/libdoc/set/rdoc/Set.html or http://www.ruby-doc.org/stdlib-2.2.0/libdoc/set/rdoc/Set.html