3 failures:
  @68 (⟨8, 0⟩): expected closing ':::' from directive on line 3 at column 0, but it's at column 1
    " :::\n"
  @72 (⟨8, 4⟩): expected token
    "\n"
  @73 (⟨9, 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
    []
    "\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
    ":::"
    <missing>
    []
    "\n"
    []
    <missing>)]