Given a 'collection of collections' like [[1, 2], [], [3], [4, 5, 6]], how would you design an iterator that hands out elements one at a time in flat order (1, 2, 3, 4, 5, 6)? It has to work directly in for-in.
A struct conforming to IteratorProtocol keeps the outer iterator and the current inner iterator as state, and next() pulls the next inner one when the current is exhausted. A Sequence wrapper's makeIterator() gives you for-in support.
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