diff --git a/AGENTS.md b/AGENTS.md
index e2d6a1643d..8ac4d169f3 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -85,12 +85,13 @@ Commands and Rust coding style: [nanvm-lib/AGENTS.md](./nanvm-lib/AGENTS.md).
 A PR implements only one feature or improvement, with minimal code changes, and
 every check above passing. Its title and description become the squash commit on
 `main`, so write them as one: a `<topic>: <short description>` title and a
-description ending in a mandatory `Changelog:` section. A PR that changes code
-adds `changelog/unreleased/<PR>.md`, named by the real PR number once the PR
-exists; a PR that only touches `todo/`, `AGENTS.md`, or other documentation
-writes `Changelog: none` instead. Breaking changes are welcome when they improve
-the API — prefix the entry with `**BREAKING CHANGES:**` and update every importer
-in the same PR.
+description ending in a mandatory `Changelog:` section. A PR that changes
+behavior or the public API adds `changelog/unreleased/<PR>.md`, named by the
+real PR number once the PR exists; a PR that doesn't — internal refactors,
+test-only changes, and PRs that only touch `todo/`, `AGENTS.md`, or other
+documentation — writes `Changelog: none` instead. Breaking changes are welcome
+when they improve the API — prefix the entry with `**BREAKING CHANGES:**` and
+update every importer in the same PR.
 
 Commit-message format and the PR checklist: [CONTRIBUTING.md](./CONTRIBUTING.md#opening-a-pull-request).
 Changelog entry rules, breaking changes, and versioning:
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index aa7dc78dd4..dcb0225ef6 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -160,9 +160,9 @@ For tool details and package-consumer setup for Claude and Codex, see
 
 A pull request implements only one feature or improvement, with minimal code
 changes. Before submitting, ensure every check above passes, delete the `todo/`
-issue file in the same pull request, and — for code changes — add a changelog
-entry as `changelog/unreleased/<PR>.md`, named by the real pull request number
-once the pull request exists (see
+issue file in the same pull request, and — when the change affects behavior or
+the public API — add a changelog entry as `changelog/unreleased/<PR>.md`, named
+by the real pull request number once the pull request exists (see
 [changelog/README.md](./changelog/README.md)). The everyday workflow around
 this is [AGENTS.md §1](./AGENTS.md#1-workflow).
 
@@ -197,9 +197,9 @@ squash, so their messages are working notes.
   The section holds exactly the list items of `changelog/unreleased/<PR>.md` —
   same Markdown subset, same `**BREAKING CHANGES:**` prefix where it applies,
   no PR link ([changelog/README.md](./changelog/README.md#entries)). A pull
-  request that needs no entry writes `Changelog: none`. The section is
-  **mandatory** either way, so a forgotten entry is a visible omission rather
-  than a silent one.
+  request that doesn't change behavior or the public API needs no entry and
+  writes `Changelog: none`. The section is **mandatory** either way, so a
+  forgotten entry is a visible omission rather than a silent one.
 
   It duplicates the entry file on purpose: the file is what today's release
   process reads, the section is what a generator reading Git history would
diff --git a/changelog/README.md b/changelog/README.md
index 11487af623..6fb2b978a5 100644
--- a/changelog/README.md
+++ b/changelog/README.md
@@ -45,9 +45,11 @@ inside the file — the file name already carries the number, and a renderer
 derives the link from it. A pull request with several entries puts them all in
 its one file, most important first.
 
-Only add entries for code changes — pull requests that only touch `todo/`,
-`AGENTS.md`, or other documentation files do not need one, and say
-`Changelog: none` in the description instead.
+Only add entries for changes that affect behavior or the public API — a pull
+request that doesn't (internal refactors, test-only changes, coverage
+improvements, and pull requests that only touch `todo/`, `AGENTS.md`, or other
+documentation files) does not need one, and says `Changelog: none` in the
+description instead.
 
 - **Keep it short.** An entry is **at most a few lines** (about three wrapped
   lines, ~250 characters) — what changed and, when it isn't obvious, why. It is a
diff --git a/deno.json b/deno.json
index 6f83506660..9440ddfcb1 100644
--- a/deno.json
+++ b/deno.json
@@ -2,7 +2,8 @@
   "tasks": {
     "fjs": "deno run --allow-read --allow-write --allow-env --allow-net --allow-sys ./fjs/module.mjs",
     "test": "deno test --allow-read --allow-env --allow-sys",
-    "cov": "deno test --allow-read --allow-env --allow-sys --coverage && deno coverage --include='.*module\\.f\\.mjs'"
+    "cov": "deno test --allow-read --allow-env --allow-sys --coverage && deno coverage --include='.*module\\.f\\.mjs'",
+    "cov-html": "deno test --allow-read --allow-env --allow-sys --coverage && deno coverage --html --include='.*module\\.f\\.mjs$'"
   },
   "fmt": {
     "indentWidth": 4,
diff --git a/fjs/types/btree/set/module.f.mjs b/fjs/types/btree/set/module.f.mjs
index 1f1abd53f3..c8ce1f4e4e 100644
--- a/fjs/types/btree/set/module.f.mjs
+++ b/fjs/types/btree/set/module.f.mjs
@@ -11,6 +11,7 @@
 import { collapseRoot } from '../types/module.f.mjs'
 import { find } from '../find/module.f.mjs'
 import { fold } from '../../list/module.f.mjs'
+import { assert } from '../../../asserts/module.f.mjs'
 
 /**
  * @template T
@@ -79,31 +80,24 @@ const nodeSet = c => g => node => {
             case 2: {
                 // insert
                 const value = g(null)
-                // TODO: remove after TSGO fix the regression.
-                // const _xl: 1|2 = x.length
                 switch (x.length) {
                     case 1: { return [[x[0], value]] }
                     case 2: { return [[x[0]], value, [x[1]]] }
                 }
-                // TODO: remove after TSGO fix the regression.
-                // See https://github.com/microsoft/typescript-go/issues/4613
-                throw 'unreachable'
             }
             case 3: {
                 // replace
                 // TODO: remove after TSGO fix the regression.
-                // const _xl: 2|5 = x.length
-                switch (x.length) {
+                const xL = x.length
+                // See https://github.com/microsoft/typescript-go/issues/4613
+                assert(xL === 2 || xL === 5)
+                switch (xL) {
                     case 2: { return [[x[0], g(x[1])]] }
                     case 5: { return [[x[0], x[1], x[2], g(x[3]), x[4]]] }
                 }
-                // TODO: remove after TSGO fix the regression.
-                throw 'unreachable'
             }
             case 4: {
                 // insert
-                // TODO: remove after TSGO fix the regression.
-                // const _xl: 2 = x.length
                 const [v0, v1] = x
                 return [[v0], v1, [g(null)]]
             }
diff --git a/package.json b/package.json
index e8261d21ed..1e75c63bb7 100644
--- a/package.json
+++ b/package.json
@@ -12,7 +12,7 @@
   "scripts": {
     "prepack": "tsc --noEmit false --emitDeclarationOnly && tsc",
     "test": "tsc && node ./fjs/module.mjs t",
-    "cov": "node --test --experimental-test-coverage --test-coverage-include=**/module.f.mjs fjs/emergent_testing/all.test.mjs",
+    "cov": "node --test --experimental-test-coverage --test-coverage-include=**/module.f.mjs",
     "start": "node ./fjs/module.mjs",
     "ci-update": "node ./fjs/module.mjs ci && node ./fjs/module.mjs r ./fjs/nanvm/update/module.f.mjs",
     "dev-update": "node ./fjs/module.mjs r ./fjs/dev/update/module.f.mjs",