--- a/Moment.thy	Wed Jan 13 15:16:59 2016 +0000
+++ b/Moment.thy	Wed Jan 13 15:22:14 2016 +0000
@@ -146,9 +146,8 @@
 lemma moment_Suc_tl:
   assumes "Suc i \<le> length s"
   shows "tl (moment (Suc i) s) = moment i s"
-  using assms unfolding moment_def
-sorry
-
+  using assms unfolding moment_def rev_take
+by (simp, metis Suc_diff_le diff_Suc_Suc drop_Suc tl_drop)
 
 lemma moment_plus:
   assumes "Suc i \<le> length s"