src/Pure/Isar/class.ML

       ((junk |--
         (Scan.optional (Scan.one Symbol.is_ascii_letter) "x" ^^ (Scan.many is_valid >> implode)
         --| junk))
       ::: Scan.repeat ((Scan.many1 is_valid >> implode) --| junk));
   in
-    explode #> scan_valids #> implode
+    raw_explode #> scan_valids #> implode
   end;
 
 val type_name = sanitize_name o Long_Name.base_name;
 
 fun define_overloaded (c, U) v (b_def, rhs) = Local_Theory.background_theory_result