improved mutabelle script to use nat for quickcheck_narrowing

--- a/src/HOL/Mutabelle/lib/Tools/mutabelle	Tue Jun 14 08:30:19 2011 +0200
+++ b/src/HOL/Mutabelle/lib/Tools/mutabelle	Tue Jun 14 08:33:51 2011 +0200
@@ -90,7 +90,7 @@
   MutabelleExtra.quickcheck_mtd (Config.put Quickcheck.tester \"exhaustive\" #> Config.put Quickcheck.finite_types false) \"exhaustive_nft\",
   MutabelleExtra.quickcheck_mtd (Config.put Quickcheck.tester \"narrowing\" #> Config.put Quickcheck.finite_types false) \"narrowing\",
   MutabelleExtra.quickcheck_mtd (Config.put Quickcheck.tester \"narrowing\" #> Config.put Quickcheck.finite_types false
-    #> Context.proof_map (Quickcheck.map_test_params (apfst (K [@{typ int}])))) \"narrowing_nat\"  
+    #> Context.proof_map (Quickcheck.map_test_params (apfst (K [@{typ nat}])))) \"narrowing_nat\"  
 (*  MutabelleExtra.nitpick_mtd *)
 ]
 *}