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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.
Please google "closure" and you should be more clear on what he have done here. This "once" function is returning another function. The call variable is set to true the first time in "once" and it will not be set back to true again since we are calling the returned function not the "once" function in the main program.