src/Pure/Thy/term_style.ML

--- a/src/Pure/Thy/term_style.ML	Sat Oct 17 16:08:30 2015 +0200
+++ b/src/Pure/Thy/term_style.ML	Sun Oct 18 17:20:20 2015 +0200
@@ -67,7 +67,7 @@
   end);
 
 fun sub_symbols (d :: s :: ss) =
-      if Symbol.is_ascii_digit d andalso not (String.isPrefix ("\<^") s)
+      if Symbol.is_ascii_digit d andalso not (Symbol.is_control s)
       then d :: "\<^sub>" :: sub_symbols (s :: ss)
       else d :: s :: ss
   | sub_symbols cs = cs;