[Accepted with modifications] SE-0045: Add scan, prefix(while:), drop(while:), and unfold to the stdlib

One idea that came out of the core team discussion was something like:

   sequence(from: 0) { $0 += 42 }

Since it returns a sequence.

It definitely occurred to me that this was kind of just a way to construct a generic iterator. Maybe a new AnyIterator (I believe there is such a thing) constructor?

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Brent Royal-Gordon
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