diff --git a/src/corpus.test.ts b/src/corpus.test.ts
index 38d67cd..9a38e98 100644
--- a/src/corpus.test.ts
+++ b/src/corpus.test.ts
@@ -1,6 +1,6 @@
 import assert from 'node:assert/strict'
 import { readFileSync, readdirSync } from 'node:fs'
-import { dirname, join } from 'node:path'
+import { basename, dirname, join } from 'node:path'
 import test from 'node:test'
 import { fileURLToPath } from 'node:url'
 
@@ -36,6 +36,21 @@ function names(root: string, extension: string): string[] {
     .sort()
 }
 
+// One kind's fixture pairs, its two tests declared with them.
+function pairedNames(root: string, first: string, second: string): string[] {
+  const kind = basename(root)
+
+  test(`${kind} pairs every ${first} with a ${second}`, () => {
+    assert.deepEqual(names(root, first), names(root, second))
+  })
+
+  test(`${kind} holds fixtures`, () => {
+    assert.ok(names(root, first).length > 0)
+  })
+
+  return names(root, first)
+}
+
 function corpusJsonPaths(): string[] {
   return readdirSync(corpusRoot, { encoding: 'utf8', recursive: true })
     .filter((name) => name.endsWith('.json'))
@@ -43,6 +58,10 @@ function corpusJsonPaths(): string[] {
     .sort()
 }
 
+test('every corpus directory is a kind the runner reads', () => {
+  assert.deepEqual(directoryNames(corpusRoot), ['errors', 'normalization', 'round-trip', 'unspellable'])
+})
+
 test('every round-trip directory emits', () => {
   assert.deepEqual(directoryNames(roundTripRoot), [...emittingDirectories].sort())
 })
@@ -147,15 +166,7 @@ for (const directory of emittingDirectories) {
   }
 }
 
-test('unspellable pairs every .json with an .error', () => {
-  assert.deepEqual(names(unspellableRoot, '.json'), names(unspellableRoot, '.error'))
-})
-
-test('unspellable holds fixtures', () => {
-  assert.ok(names(unspellableRoot, '.json').length > 0)
-})
-
-for (const name of names(unspellableRoot, '.json')) {
+for (const name of pairedNames(unspellableRoot, '.json', '.error')) {
   test(`unspellable/${name} is refused with the error it names`, () => {
     const parsed: unknown = JSON.parse(readFileSync(join(unspellableRoot, `${name}.json`), 'utf8'))
     assert.ok(isAdfDocument(parsed), `${name}.json is not an ADF document`)
@@ -165,15 +176,7 @@ for (const name of names(unspellableRoot, '.json')) {
   })
 }
 
-test('normalization pairs every .md with a .json', () => {
-  assert.deepEqual(names(normalizationRoot, '.md'), names(normalizationRoot, '.json'))
-})
-
-test('normalization holds fixtures', () => {
-  assert.ok(names(normalizationRoot, '.md').length > 0)
-})
-
-for (const name of names(normalizationRoot, '.md')) {
+for (const name of pairedNames(normalizationRoot, '.md', '.json')) {
   test(`normalization/${name} parses to the document beside it`, () => {
     const expected: unknown = JSON.parse(readFileSync(join(normalizationRoot, `${name}.json`), 'utf8'))
     assert.ok(isAdfDocument(expected), `${name}.json is not an ADF document`)
@@ -183,15 +186,7 @@ for (const name of names(normalizationRoot, '.md')) {
   })
 }
 
-test('errors pairs every .md with an .error', () => {
-  assert.deepEqual(names(errorsRoot, '.md'), names(errorsRoot, '.error'))
-})
-
-test('errors holds fixtures', () => {
-  assert.ok(names(errorsRoot, '.md').length > 0)
-})
-
-for (const name of names(errorsRoot, '.md')) {
+for (const name of pairedNames(errorsRoot, '.md', '.error')) {
   test(`errors/${name} is refused with the error it names`, () => {
     const result = markdownToAdf(readFileSync(join(errorsRoot, `${name}.md`), 'utf8'))
     assert.ok(!result.ok, result.ok ? `built ${JSON.stringify(result.value)}` : '')
diff --git a/src/markdown/emit/line-escaping.ts b/src/markdown/emit/line-escaping.ts
index 5591e4b..c02fc64 100644
--- a/src/markdown/emit/line-escaping.ts
+++ b/src/markdown/emit/line-escaping.ts
@@ -29,6 +29,7 @@ type EmittedRun = { canClose: boolean; canOpen: boolean; character: string; deli
 
 const delimiters = ['*', '_', '`', '~']
 
+// The `:` keeps a `[label]: url` line escaped: unescaped, the parser swallows it as a link reference definition.
 const followsLinkText = /[([:]/
 
 export function assembleInlineLine(segments: readonly InlineSegment[], container: LineContainer): AssembledLine {
diff --git a/src/markdown/parse/blocks.test.ts b/src/markdown/parse/blocks.test.ts
index 96966ee..1bfdb61 100644
--- a/src/markdown/parse/blocks.test.ts
+++ b/src/markdown/parse/blocks.test.ts
@@ -2,21 +2,14 @@ import assert from 'node:assert/strict'
 import test from 'node:test'
 
 import type { LinkDefinition } from './link-reference-definitions.ts'
-import type { ParsedBlocks } from './blocks.ts'
 import { parseBlocks } from './blocks.ts'
 
-function walk(markdown: string): ParsedBlocks {
-  const result = parseBlocks(markdown)
-  assert.ok(result.ok, result.ok ? '' : `${result.error.code}: ${result.error.message}`)
-  return result.value
-}
-
 function definitions(markdown: string): [string, LinkDefinition][] {
-  return [...walk(markdown).definitions]
+  return [...parseBlocks(markdown).definitions]
 }
 
 function kinds(markdown: string): string[] {
-  return walk(markdown).blocks.map((block) => block.kind)
+  return parseBlocks(markdown).blocks.map((block) => block.kind)
 }
 
 test('keeps the link reference definitions a paragraph gives up, the first of a label winning', () => {
@@ -51,3 +44,8 @@ test('swallows an HTML block to the end condition its start sets', () => {
   assert.deepEqual(kinds('<div>\nx\n'), ['html'])
   assert.deepEqual(kinds('Part.\n<div>\n'), ['paragraph', 'html'])
 })
+
+test('carries a claimed line as the block it opens, the refusal the node layer builds', () => {
+  assert.deepEqual(kinds(':::\nPart.\n'), ['claim', 'paragraph'])
+  assert.deepEqual(kinds('Part.\n| x |\n'), ['paragraph', 'claim'])
+})
diff --git a/src/markdown/parse/blocks.ts b/src/markdown/parse/blocks.ts
index 2af8cf5..01a8ce6 100644
--- a/src/markdown/parse/blocks.ts
+++ b/src/markdown/parse/blocks.ts
@@ -1,14 +1,16 @@
 import type { LinkDefinition } from './link-reference-definitions.ts'
 import type { OpenHtmlBlock } from './html-blocks.ts'
 import { atxHeading, claimsDirectiveLine, claimsPipeLine, closingCodeFence, isThematicBreak, openingCodeFence, setextHeadingLevel } from '../commonmark-grammar.ts'
-import { failure, success, type Result } from '../../result.ts'
 import { openingHtmlBlock } from './html-blocks.ts'
 import { readLinkDefinitions } from './link-reference-definitions.ts'
 
+export type ClaimedConstruct = 'directive' | 'pipe-table'
+
 export type LeafBlock =
+  | { construct: ClaimedConstruct; kind: 'claim' }
+  | { construct: string; kind: 'html' }
   | { kind: 'code'; language: string; text: string }
   | { kind: 'heading'; level: number; text: string }
-  | { kind: 'html'; name: string }
   | { kind: 'paragraph'; text: string }
   | { kind: 'rule' }
 
@@ -21,41 +23,51 @@ const indentedCodeColumns = 4
 const largestOpenerIndentation = 3
 const tabStop = 4
 
-export function parseBlocks(markdown: string): Result<ParsedBlocks> {
+export function parseBlocks(markdown: string): ParsedBlocks {
   const lines = normalizeInput(markdown).split('\n')
   const walk: Walk = { blocks: [], definitions: new Map(), paragraph: [] }
   let index = 0
   while (index < lines.length) {
     const line = lines[index] ?? ''
-    index += 1
     if (blankLine.test(line)) {
       closeParagraph(walk)
+      index += 1
       continue
     }
     if (leadingColumns(line) >= indentedCodeColumns && walk.paragraph.length === 0) {
-      index = readIndentedCode(walk, lines, index - 1)
+      index = readIndentedCode(walk, lines, index)
       continue
     }
-    const opener = removeColumns(line, largestOpenerIndentation)
-    const claim = claimedLine(walk, opener)
-    if (claim !== undefined) return claim
-    if (readLineBlock(walk, opener)) continue
-    const fence = openingCodeFence(opener)
-    if (fence !== undefined) {
-      closeParagraph(walk)
-      index = readFencedCode(walk, lines, index, fence, leadingColumns(line))
-      continue
-    }
-    const html = openingHtmlBlock(opener, walk.paragraph.length > 0)
-    if (html !== undefined) {
-      closeParagraph(walk)
-      index = readHtmlBlock(walk, lines, index - 1, html)
+    const opened = openBlock(walk, lines, index, line)
+    if (opened !== undefined) {
+      index = opened
       continue
     }
     walk.paragraph.push(line.replace(/^[ \t]+/, ''))
+    index += 1
   }
   closeParagraph(walk)
-  return success({ blocks: walk.blocks, definitions: walk.definitions })
+  return { blocks: walk.blocks, definitions: walk.definitions }
+}
+
+function openBlock(walk: Walk, lines: readonly string[], index: number, line: string): number | undefined {
+  const opener = removeColumns(line, largestOpenerIndentation)
+  const claimed = claimedConstruct(opener)
+  if (claimed !== undefined) {
+    closeParagraph(walk)
+    walk.blocks.push({ construct: claimed, kind: 'claim' })
+    return index + 1
+  }
+  if (readLineBlock(walk, opener)) return index + 1
+  const fence = openingCodeFence(opener)
+  if (fence !== undefined) {
+    closeParagraph(walk)
+    return readFencedCode(walk, lines, index + 1, fence, leadingColumns(line))
+  }
+  const html = openingHtmlBlock(opener, walk.paragraph.length > 0)
+  if (html === undefined) return undefined
+  closeParagraph(walk)
+  return readHtmlBlock(walk, lines, index, html)
 }
 
 // The document's last line ending closes its line rather than opening an empty one.
@@ -66,13 +78,9 @@ function normalizeInput(markdown: string): string {
     .replace(/\n$/, '')
 }
 
-function claimedLine(walk: Walk, opener: string): Result<ParsedBlocks> | undefined {
-  const directive = claimsDirectiveLine(opener)
-  if (!directive && !claimsPipeLine(opener)) return undefined
-  closeParagraph(walk)
-  const path = ['content', walk.blocks.length]
-  if (directive) return failure('malformed-directive', 'the line claims a directive and parses as none', path)
-  return failure('malformed-pipe-table', 'the line claims a pipe table and parses as none', path)
+function claimedConstruct(opener: string): ClaimedConstruct | undefined {
+  if (claimsDirectiveLine(opener)) return 'directive'
+  return claimsPipeLine(opener) ? 'pipe-table' : undefined
 }
 
 // A setext underline over a paragraph the definitions emptied is no heading: it opens the next block.
@@ -139,7 +147,7 @@ function readHtmlBlock(walk: Walk, lines: readonly string[], start: number, html
     index += 1
     if (html.closer !== undefined && html.closer.test(line)) break
   }
-  walk.blocks.push({ kind: 'html', name: html.name })
+  walk.blocks.push({ construct: html.construct, kind: 'html' })
   return index
 }
 
diff --git a/src/markdown/parse/html-blocks.ts b/src/markdown/parse/html-blocks.ts
index 76a1e91..ad5ef61 100644
--- a/src/markdown/parse/html-blocks.ts
+++ b/src/markdown/parse/html-blocks.ts
@@ -1,6 +1,6 @@
-export type OpenHtmlBlock = { closer: RegExp | undefined; name: string }
+export type OpenHtmlBlock = { closer: RegExp | undefined; construct: string }
 
-type HtmlBlockCondition = { closer: RegExp | undefined; interrupts: boolean; name: string | undefined; start: RegExp }
+type HtmlBlockCondition = { closer: RegExp | undefined; construct: string | undefined; interrupts: boolean; start: RegExp }
 
 // CommonMark 0.31.2, HTML blocks: the tag names start condition 6 lists.
 const blockTagNames =
@@ -10,19 +10,19 @@ const completeTag = new RegExp(`^(?:<[A-Za-z][A-Za-z0-9-]*${attributeSource}*[ \
 const tagName = /^<\/?([A-Za-z][A-Za-z0-9-]*).*$/
 
 const conditions: HtmlBlockCondition[] = [
-  { closer: /<\/(?:pre|script|style|textarea)>/i, interrupts: true, name: undefined, start: /^<(?:pre|script|style|textarea)(?:[ \t>]|$)/i },
-  { closer: /-->/, interrupts: true, name: 'an HTML comment', start: /^<!--/ },
-  { closer: /\?>/, interrupts: true, name: 'an HTML processing instruction', start: /^<\?/ },
-  { closer: />/, interrupts: true, name: 'an HTML declaration', start: /^<![A-Za-z]/ },
-  { closer: /\]\]>/, interrupts: true, name: 'a CDATA section', start: /^<!\[CDATA\[/ },
-  { closer: undefined, interrupts: true, name: undefined, start: new RegExp(`^</?(?:${blockTagNames})(?:[ \\t>]|/>|$)`, 'i') },
-  { closer: undefined, interrupts: false, name: undefined, start: completeTag },
+  { closer: /<\/(?:pre|script|style|textarea)>/i, construct: undefined, interrupts: true, start: /^<(?:pre|script|style|textarea)(?:[ \t>]|$)/i },
+  { closer: /-->/, construct: 'an HTML comment', interrupts: true, start: /^<!--/ },
+  { closer: /\?>/, construct: 'an HTML processing instruction', interrupts: true, start: /^<\?/ },
+  { closer: />/, construct: 'an HTML declaration', interrupts: true, start: /^<![A-Za-z]/ },
+  { closer: /\]\]>/, construct: 'a CDATA section', interrupts: true, start: /^<!\[CDATA\[/ },
+  { closer: undefined, construct: undefined, interrupts: true, start: new RegExp(`^</?(?:${blockTagNames})(?:[ \\t>]|/>|$)`, 'i') },
+  { closer: undefined, construct: undefined, interrupts: false, start: completeTag },
 ]
 
 export function openingHtmlBlock(line: string, interrupting: boolean): OpenHtmlBlock | undefined {
   for (const condition of conditions) {
     if ((interrupting && !condition.interrupts) || !condition.start.test(line)) continue
-    return { closer: condition.closer, name: condition.name ?? line.replace(tagName, '<$1>') }
+    return { closer: condition.closer, construct: condition.construct ?? line.replace(tagName, '<$1>') }
   }
   return undefined
 }
diff --git a/src/markdown/parse/markdown-to-adf.test.ts b/src/markdown/parse/markdown-to-adf.test.ts
index be8a3ec..5274de6 100644
--- a/src/markdown/parse/markdown-to-adf.test.ts
+++ b/src/markdown/parse/markdown-to-adf.test.ts
@@ -107,6 +107,7 @@ test('refuses the raw HTML no element mapping carries', () => {
   assert.equal(code(markdownToAdf('<pre>\nx\n</pre>\n')), 'unmappable-html')
   assert.equal(code(markdownToAdf('<span foo="bar">\n')), 'unmappable-html')
   assert.deepEqual(path(markdownToAdf('Part.\n\n<div>\n')), ['content', 1])
+  assert.equal(code(markdownToAdf('<div>\nx\n\n:::\n')), 'unmappable-html')
 })
 
 test('swallows an HTML block ahead of the claim a line inside it would make', () => {
diff --git a/src/markdown/parse/markdown-to-adf.ts b/src/markdown/parse/markdown-to-adf.ts
index 06709f2..8474ba4 100644
--- a/src/markdown/parse/markdown-to-adf.ts
+++ b/src/markdown/parse/markdown-to-adf.ts
@@ -1,13 +1,11 @@
 import type { AdfDocument, AdfNode } from '../../adf/document.ts'
-import type { LeafBlock } from './blocks.ts'
+import type { ClaimedConstruct, LeafBlock } from './blocks.ts'
 import { failure, success, type ConvertErrorPath, type Result } from '../../result.ts'
 import { parseBlocks } from './blocks.ts'
 
 export function markdownToAdf(markdown: string): Result<AdfDocument> {
-  const parsed = parseBlocks(markdown)
-  if (!parsed.ok) return parsed
   const content: AdfNode[] = []
-  for (const [index, block] of parsed.value.blocks.entries()) {
+  for (const [index, block] of parseBlocks(markdown).blocks.entries()) {
     const node = blockNode(block, ['content', index])
     if (!node.ok) return node
     content.push(node.value)
@@ -16,13 +14,19 @@ export function markdownToAdf(markdown: string): Result<AdfDocument> {
 }
 
 function blockNode(block: LeafBlock, path: ConvertErrorPath): Result<AdfNode> {
+  if (block.kind === 'claim') return claimFailure(block.construct, path)
   if (block.kind === 'code') return success(codeBlockNode(block.language, block.text))
   if (block.kind === 'heading') return success(withContent({ attrs: { level: block.level }, type: 'heading' }, block.text))
-  if (block.kind === 'html') return failure('unmappable-html', `no element mapping carries ${block.name}`, path)
+  if (block.kind === 'html') return failure('unmappable-html', `no ADF node carries ${block.construct}`, path)
   if (block.kind === 'paragraph') return success(withContent({ type: 'paragraph' }, block.text))
   return success({ type: 'rule' })
 }
 
+function claimFailure(construct: ClaimedConstruct, path: ConvertErrorPath): Result<AdfNode> {
+  if (construct === 'directive') return failure('malformed-directive', 'the line claims a directive and parses as none', path)
+  return failure('malformed-pipe-table', 'the line claims a pipe table and parses as none', path)
+}
+
 function codeBlockNode(language: string, text: string): AdfNode {
   const node: AdfNode = language === '' ? { type: 'codeBlock' } : { attrs: { language }, type: 'codeBlock' }
   return text === '' ? node : { ...node, content: [{ text, type: 'text' }] }
diff --git a/todo.md b/todo.md
index 6135f99..5e79828 100644
--- a/todo.md
+++ b/todo.md
@@ -210,7 +210,10 @@ detail is settled at its own milestone.
         rather than editing them.
   - [ ] **3c — The container blocks.** Blockquote, bullet and ordered list: the continuation a
         marker's width sets, lazy continuation, and the tightness ADF does not record — `> `
-        repeated being two bytes a level, so this is the cheapest way to reach §11's 500.
+        repeated being two bytes a level, so this is the cheapest way to reach §11's 500. 3b's leaf
+        readers scan the physical line themselves, so a container re-cuts the walk rather than adding
+        to it: the open containers' prefix comes off the line first and the readers take one line at a
+        time, `LeafBlock` renamed with the union they join.
         **Settled** (the maintainer, 2026-08-27): a claimed line ends lazy continuation, so a
         closing fence on the line after a blockquote's open paragraph closes its container instead
         of continuing the paragraph CommonMark would fold it into. Claiming at block level is
@@ -244,7 +247,8 @@ detail is settled at its own milestone.
   - [ ] **3f — The directive grammar.** The three forms — inline `:name[content]{attrs}`,
         container `:::name arg {attrs}`, leaf `::name arg {attrs}` — the attribute grammar with
         its quoting and escapes, the fence-length and nesting rules, and the malformed list
-        `spec/flavour.md` spells, each a named error.
+        `spec/flavour.md` spells, each a named error. `corpus.test.ts`'s `fenceNestingFault` stays a
+        second reading of the fence rule over emitted bytes: the double entry is the check.
         **Settled** (the maintainer, 2026-08-27): the code span, the entity and raw HTML bind
         first in input, as 2e3 already assumed of the emitted side — a raw `` ` ``, `&`, `<` or
         `|` inside `{attrs}` breaks the directive and is a named error, the author writing the
@@ -281,7 +285,8 @@ detail is settled at its own milestone.
         named error, a carry inside a mark spelling another, and the three carve-outs' escapes
         reading as the literal text they hold.
   - [ ] **3k — The CommonMark spec suite.** Checked in at `corpus/commonmark-spec/`, pinned to
-        the version it ships, `corpus/README.md` gaining the kind.
+        the version it ships — the one `html-blocks.ts` names for its start conditions —
+        `corpus/README.md` gaining the kind.
         **Settled** (the maintainer, 2026-08-27): three checks an example must pass, the reference
         HTML each ships read as corpus data — which adds no format and no direction (§1). §2's
         canonical fixpoint: a named error, or markdown that parses and emits to itself byte for