Demonstration 1 of 3
Sections glue, or they obstruct.
Glued, or refused
Agreement between sources is checkable where they overlap, and a failure carries a location as well as a size.
How to read this figure
The five patches of the cover, U₀ to U₄, each its own colour.
The base is a Klein bottle. Go once round and the fibre comes back reversed.
The seam whose disagreement cannot be absorbed.
The patch beyond it, lifted clear: no section extends across.
Agreement between sources is checkable where they overlap, and a failure carries a location as well as a size. What is measured is the disagreement on each overlap: two sources on the same ground reporting different numbers. On a cover whose nerve is a cycle a global section exists exactly when those disagreements sum to zero round it, so that sum is the class. Which seam is at fault is answered separately, by the disagreement that stands out against the scale the other four set.