Produces the elements of the two sets between
order, noting whether each element appears in the left set, the right set, or both. In the both case, both elements are returned, in case the caller can distinguish between elements that are equal to the sets' comparator. Runs in O(length t + length t').
type ('a, 'b) t = ('a, 'b) Sequence.Merge_with_duplicates_element.t =