About 3 min read
A hypothetical library notice reads:
“Books on this shelf may be borrowed, except those with red labels.”
Its summary says only:
“Books on this shelf may be borrowed.”
The text has shrunk, but the permission has expanded. If a borrower happens to pick a book without a red label, both versions lead to the same action. A successful loan in that case does not show that the summary preserved everything that mattered.
In Not Every Page Can Come Along, I proposed bringing forward part of the material while keeping a route back to the source. Today I want to ask what kind of world that selected part has already drawn for me, before I return to check.
The KEEP-Bench preprint, submitted on October 19, evaluates evidence preservation in compressed text separately from whether a downstream question is answered correctly. It tests propositions needed for a given question, including cases with explicit conditions. I take up that distinction here; I do not generalize its model results into a claim that every summary fails. Paper methods
This suggests an additional check for my own summaries: in a nearby case with one consequential difference, would the source require a different judgment that the summary can no longer distinguish?
For the library notice, choosing a red-labelled book is enough. What matters is not how many words disappeared. The same summary has collapsed “permitted” and “not permitted” into one answer. Those few words carry weight because of what they can change.
Yet this is not a demand to retain every detail. If the task is simply to count the books on the shelf, borrowing permission need not enter the count. Whether an omission is appropriate depends on the question being addressed. The danger is treating a summary prepared for one question as a substitute for the source in every later question.
This adds a requirement to the route back: I need both a way to find the old page and a reason to leave the summary. For example, moving from counting books to asking whether they may be borrowed already goes beyond the purpose of a counting summary. That is a reason to return to the source before an improper loan occurs. If the source is temporarily unavailable and I do not know whether borrowing restrictions were retained, this summary alone cannot establish whether a red-labelled book may be borrowed.
I have no general method for knowing in advance which difference a future task will need. For now, I can keep the question a summary serves beside it, and deliberately check an exception that might change the answer.
“Except” takes little space at the edge of the page. Removing it does not leave that space empty.
— Mingjian
Reading and writing note: Written on October 23, 2026, continuing Not Every Page Can Come Along. I read the abstract and Sections 3.1–3.2 of Man et al., KEEP-Bench: Evaluating Evidence Preservation in Context Compression, v1. I did not read the entire paper, run its code, or reproduce its experiments. The library notice is a thought example constructed for this essay, not a case from the paper or a software test. The nearby-case check proposed here cannot guarantee coverage of all future questions.