The Scan<T> implementation throws when an empty sequence is found. I believe the correct behavior should be to yield break in that case.

This would mean a different behavior from the Scan<T, TState> implementation, but it is already different. Without a seed, an IEnumerable of size N will yield an IEnumerable of the same size, while the version with seed adds an extra element.

So making a 0-length IEnumerable result in a 0-length IEnumerable seems like the right behavior to me.

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