added [simp]

--- a/src/HOL/List.thy	Sun Oct 15 21:30:21 2017 +0200
+++ b/src/HOL/List.thy	Mon Oct 16 08:00:28 2017 +0200
@@ -2042,7 +2042,7 @@
 lemma drop_Suc: "drop (Suc n) xs = drop n (tl xs)"
 by(cases xs, simp_all)
 
-lemma hd_take: "j > 0 \<Longrightarrow> hd (take j xs) = hd xs"
+lemma hd_take[simp]: "j > 0 \<Longrightarrow> hd (take j xs) = hd xs"
 by (metis gr0_conv_Suc list.sel(1) take.simps(1) take_Suc)
 
 lemma take_tl: "take n (tl xs) = tl (take (Suc n) xs)"