2 failures:
  @71 (⟨8, 0⟩): expected closing '::::' for directive from line 3
    ":::: a\n\nx\n"
  @81 (⟨11, 0⟩): expected closing '::::' for directive from line 8
    ""

Final stack:
  [(Lean.Doc.Syntax.para
    "para{"
    [(Lean.Doc.Syntax.text
      (str
       "\"Here's some error recovery for directives.\""))]
    "}")
   (Lean.Doc.Syntax.directive
    "::::"
    `foo
    [(Lean.Doc.Syntax.anon
      (Lean.Doc.Syntax.arg_num (num "5")))]
    "\n"
    [(Lean.Doc.Syntax.para
      "para{"
      [(Lean.Doc.Syntax.text (str "\"Indeed.\""))]
      "}")
     (Lean.Doc.Syntax.para
      "para{"
      [(Lean.Doc.Syntax.text (str "\"It is.\""))]
      "}")]
    <missing>)
   (Lean.Doc.Syntax.directive
    "::::"
    `a
    []
    "\n"
    [(Lean.Doc.Syntax.para
      "para{"
      [(Lean.Doc.Syntax.text (str "\"x\""))
       (Lean.Doc.Syntax.linebreak
        "line!"
        (str "\"\\n\""))]
      "}")]
    <missing>)]