3 failures:
  @73 (⟨9, 3⟩): expected token
    "\n"
  @74 (⟨10, 0⟩): expected closing ':::' for directive from line 9
    ""
  @74 (⟨10, 0⟩): expected closing '::::' for directive from line 3
    ""

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