Function fetch_gap 

fn fetch_gap(
    prior: ObjectLocation,
    next: ObjectLocation,
    order: GroupOrder,
) -> Vec<(ObjectLocation, ObjectLocation)>

The Locations strictly between two consecutive entries of a FETCH stream, as inclusive ranges.

draft-23 Section 3.2.2: “When two consecutive objects A and B are in different groups, a gap represents multiple logical gaps: the tail of A.Group; all skipped groups; and {B.Group, 0} through B, exclusive.” Groups arrive in the request’s Group Order and, Section 3.2.1, “Within each group, objects are sent in Object ID order”. An entry that does not move forward in that order leaves no gap.