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Chapter 10: challenge 1 '.do'?

OK, so I’ve been scratching my head over the first challenge in chapter 10 and finally had to look at the solution, which involves a “.do” operator. A what now? I’ve searched back through the book and it looks like “.do” hasn’t been introduced so far, so feeling a little “erm, oh” about this particular challenge. Did I miss something?

What does ‘.do’ do? And how are we supposed to have come up with that solution if we haven’t heard of it yet?