fixed proof (cf. 40501bb2d57c);

--- a/src/HOL/Extraction/Euclid.thy	Tue Jul 07 17:39:51 2009 +0200
+++ b/src/HOL/Extraction/Euclid.thy	Tue Jul 07 20:16:06 2009 +0200
@@ -189,7 +189,7 @@
       assume pn: "p \<le> n"
       from `prime p` have "0 < p" by (rule prime_g_zero)
       then have "p dvd n!" using pn by (rule dvd_factorial)
-      with dvd have "p dvd ?k - n!" by (rule nat_dvd_diff)
+      with dvd have "p dvd ?k - n!" by (rule dvd_diff_nat)
       then have "p dvd 1" by simp
       with prime show False using prime_nd_one by auto
     qed