src/Pure/Isar/token.ML

   (case kind of
     String => x |> quote o escape "\""
   | AltString => x |> enclose "`" "`" o escape "`"
   | Verbatim => x |> enclose "{*" "*}"
   | Comment => x |> enclose "(*" "*)"
-  | Malformed => space_implode " " (explode x)
   | Sync => ""
   | EOF => ""
   | _ => x);
 
 fun text_of tok =