Lollipop Sequence Space Comparisons
a is older than b on span N if:
- a < 0 and a < b, or
- a > 0, a < b, and (b - a) < N/4, or
- a > 0, b > 0, and (a - b) > N/4
- [makes -N/2 the oldest seq number]
When receiving an old number, must inform sender of latest seq number