[flqn]

Regarding the comment on discriminated unions, I've actually found the opposite with the new ability to have separate types for sentinels. Before, since both iterators had to be the same type, the end iterator needed some special casing to know it was the end. Allowing it to be a separate type meant all the logic for traversing the underlying sequence could be in the iterator type, and the sentinel could be an empty struct like a tag type used for comparisons only.