Answer the second stability pass: the emitter's own refusal, and the comparator's third arm

src/markdown/parse/markdown-to-adf.test.ts

@@ -148,7 +148,6 @@ test('reads the three directive forms into the nodes the tables name', () => {
   assert.deepEqual(content(markdownToAdf('Part:hardBreak{}.\n')), [{ content: [text('Part'), hardBreak(), text('.')], type: 'paragraph' }])
 })
 
-// The emitter's plain-versus-directive choice, read backwards: only the form it picks parses.
 test('names the directive form a node CommonMark spells refuses', () => {
   const named = (type: string): string => `unsupported-node-shape: ${type} takes the CommonMark spelling, not the directive form`
   assert.equal(content(markdownToAdf('::rule\n')), named('rule'))
@@ -160,6 +159,12 @@ test('names the directive form a node CommonMark spells refuses', () => {
   assert.deepEqual(content(markdownToAdf('::::bulletList\n:::listItem\n---\n:::\n::::\n')), [bulletList(item({ type: 'rule' }))])
 })
 
+// The spelling the emitter refuses gives the emitter's own error, never a second name for it.
+test('gives back the refusal the CommonMark spelling itself raises', () => {
+  const nested = '::::::::bulletList\n:::::::listItem\n---\n\n::::::bulletList\n:::::listItem\n---\n\n::::bulletList\n:::listItem\n---\n:::\n::::\n:::::\n::::::\n:::::::\n::::::::\n'
+  assert.equal(code(markdownToAdf(nested)), 'unspelled-block-separation')
+})
+
 test('names the directive name no node reads back to', () => {
   assert.equal(code(markdownToAdf(':::widget info\nx\n:::\n')), 'unknown-directive-name')
   assert.equal(content(markdownToAdf('::widget\n')), 'unknown-directive-name: the directive name widget reads back to no node')