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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.
That's bitwise AND.
See here: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Bitwise_AND
In this case it's functioning as a shorthand to check to see if a number is odd. In binary, odd integers always end in 1, while even integers always end in 0. So if n is even then n&1 evaluates to 0, and if n is odd then n&1 evaluates to 1.