This is the first fix surfaced by the pedagogy loop (#5), run across all 23 lessons three times. The headline is the non-change: 22 of 23 lessons passed the teaching rubric 8/8 every pass. The collection teaches well. One majority-agreed gap remained — and it's a precise, small one.

cuckoo-filter was flagged on a single rubric item, "lands the transfer", by 2 of 3 independent evaluators. Their agreement was specific: the coda distills the cuckoo filter's own mechanism ("four moves: hash, place, evict, forget") and lists its neighbours, but never lifts one principle to carry to other problems — the thing a good close leaves you with. Both evaluators even named the same candidate the lesson had already built but not elevated: recovering a location by XOR instead of storing it.

The change adds a single paragraph to the coda, right after "There is very little here. That is the point." It takes the XOR trick the lesson already taught — a fingerprint's alternate bucket is recomputed, i₂ = i₁ ⊕ hash(fp), never stored — and names the transferable idea inside it: derive what you can instead of keeping it. Nothing else moves; the style mirrors the existing body block (IBM Plex Serif, the same width and weight), with the formula in the family's JetBrains Mono.