Since batches are about splitting an IEnumerable into fixed sizes, every batch should know how many items it has, in addition to the original order of the source enumerable. Size + order preservation = IList.

This should break little (if any) existing code, because IList inherits IEnumerable. Asides from the signature, the only code change that needs to be made is for the underlying bucket store to be a List instead of an Array (which is the more LINQy approach anyways).

Or at least IReadOnlyCollection.

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