--- a/Nominal/Fv.thy	Thu Mar 11 11:32:37 2010 +0100
+++ b/Nominal/Fv.thy	Thu Mar 11 11:41:27 2010 +0100
@@ -174,7 +174,7 @@
       in
         if arg_no mem args_in_bn then 
           (if is_rec_type dt then
-            (if body_index dt = ith_dtyp then fvbn $ x else error "fv_bn: not good")
+            (if body_index dt = ith_dtyp then fvbn $ x else error "fv_bn: recursive argument, but wrong datatype.")
           else @{term "{} :: atom set"}) else
         if is_atom thy ty then mk_single_atom x else
         if is_atom_set thy ty then mk_atoms x else