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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 personally think that is best use do filter and includes since these are functions that can (and mostlikely will) be otmized by the interpreter/compiler.
What you did is still valid tho, but is better to use Set instead of creating a new array with the element/boolean formant