Name the position a directive name the other one spells belongs to

src/markdown/parse/directive-marks.ts

@@ -1,4 +1,5 @@
 import type { AdfMark } from '../../adf/document.ts'
+import type { ConvertFault } from '../../result.ts'
 import type { DirectiveAttributes } from '../directive-syntax.ts'
 import type { MarkSpelling } from '../mark-spellings.ts'
 import { failure, success, type ConvertErrorPath, type Result } from '../../result.ts'
@@ -15,6 +16,12 @@ export function readDirectiveMark(name: string, attributes: DirectiveAttributes,
   return success(Object.keys(attrs.value).length === 0 ? { type: name } : { attrs: attrs.value, type: name })
 }
 
+export function markSpellingFault(name: string): ConvertFault | undefined {
+  const spelling = markSpelling(name)
+  if (spelling === undefined) return undefined
+  return { code: 'unsupported-node-shape', message: `${name} is spelled ${markdownForm(spelling) ?? `:${name}[…]`}, never as a block directive` }
+}
+
 function markdownForm(spelling: MarkSpelling): string | undefined {
   switch (spelling.kind) {
     case 'code':