> an implicit requirement that back pressure from the consumer should be handled (i.e. if whoever's iterating through "async for fut in as_completed(...)" is too slow, then the tasks should pause until it catches up)

No, I don't think it is required or desired to be handled.

My initial sketch was imprecise: it's better to asynchronously yield task results, not "completed task" futures. This way, no additional buffer needed, all error handling can be consolidated inside `igather()`, and that's actually more compatible with `asyncio.gather()`.

I.e, instead of
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yield next_fut
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use
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yield await next_fut
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