follow up of 0819931d652d -- put right induction rule in the old data structure, repairs 'HOL-Proof'-based sessions

--- a/src/HOL/Tools/BNF/bnf_lfp_compat.ML	Mon Feb 17 22:54:38 2014 +0100
+++ b/src/HOL/Tools/BNF/bnf_lfp_compat.ML	Tue Feb 18 01:11:25 2014 +0100
@@ -132,7 +132,7 @@
       else
         ((fp_sugars0, (NONE, NONE)), lthy);
 
-    val {co_inducts = [induct], ...} :: _ = fp_sugars;
+    val {common_co_inducts = [induct], ...} :: _ = fp_sugars;
     val inducts = map (the_single o #co_inducts) fp_sugars;
 
     fun mk_dtyp [] (TFree a) = Datatype_Aux.DtTFree a