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I think it only iterates over the collection once.

That and they're collection agnostic. I highly recommend watching Rich Hickey's talk for more background: https://www.youtube.com/watch?v=6mTbuzafcII

Then what happens to operations that access the collection? like third argument in JavaScript's native forEach?

That's what a zipper [1] is good for. It gives you access to the rest of a collection relative to the current position.

It's not supported. But if you're doing things right, you shouldn't need it.