use empty binding rather than "*" for default

--- a/src/HOL/Codatatype/Tools/bnf_wrap.ML	Wed Sep 05 08:32:59 2012 +0200
+++ b/src/HOL/Codatatype/Tools/bnf_wrap.ML	Wed Sep 05 09:31:31 2012 +0200
@@ -40,7 +40,7 @@
 val split_asmN = "split_asm";
 val weak_case_cong_thmsN = "weak_case_cong";
 
-val no_binder = @{binding "*"};
+val no_binder = @{binding ""};
 val fallback_binder = @{binding _};
 
 fun pad_list x n xs = xs @ replicate (n - length xs) x;