diff --git a/src/HOL/Library/Library.thy b/src/HOL/Library/Library.thy
--- a/src/HOL/Library/Library.thy
+++ b/src/HOL/Library/Library.thy
@@ -1,100 +1,101 @@
 (*<*)
 theory Library
 imports
   AList
   Adhoc_Overloading
   BigO
   BNF_Axiomatization
   BNF_Corec
   Bourbaki_Witt_Fixpoint
   Char_ord
   Code_Cardinality
   Code_Lazy
   Code_Test
   Combine_PER
   Complete_Partial_Order2
   Conditional_Parametricity
   Confluence
   Confluent_Quotient
   Countable
   Countable_Complete_Lattices
   Countable_Set_Type
   Debug
   Diagonal_Subsequence
   Discrete
   Disjoint_Sets
   Disjoint_FSets
   Dlist
   Dual_Ordered_Lattice
   Equipollence
   Extended
   Extended_Nat
   Extended_Nonnegative_Real
   Extended_Real
   Finite_Map
   Float
   FSet
   FuncSet
   Function_Division
   Fun_Lexorder
   Going_To_Filter
   Groups_Big_Fun
   Indicator_Function
   Infinite_Set
   Interval
   Interval_Float
   IArray
   Landau_Symbols
   Lattice_Algebras
   Lattice_Constructions
   Linear_Temporal_Logic_on_Streams
   ListVector
   Lub_Glb
   Mapping
   Monad_Syntax
   More_List
   Multiset_Order
+  NList
   Nonpos_Ints
   Numeral_Type
   Omega_Words_Fun
   Open_State_Syntax
   Option_ord
   Order_Continuity
   Parallel
   Pattern_Aliases
   Periodic_Fun
   Poly_Mapping
   Power_By_Squaring
   Preorder
   Product_Plus
   Quadratic_Discriminant
   Quotient_List
   Quotient_Option
   Quotient_Product
   Quotient_Set
   Quotient_Sum
   Quotient_Syntax
   Quotient_Type
   Ramsey
   Reflection
   Rewrite
   Saturated
   Set_Algebras
   Set_Idioms
   Signed_Division
   State_Monad
   Stream
   Sorting_Algorithms
   Sublist
   Sum_of_Squares
   Transitive_Closure_Table
   Tree_Multiset
   Tree_Real
   Type_Length
   Uprod
   While_Combinator
   Word
   Z2
 begin
 end
 (*>*)
diff --git a/src/HOL/Library/NList.thy b/src/HOL/Library/NList.thy
new file mode 100644
--- /dev/null
+++ b/src/HOL/Library/NList.thy
@@ -0,0 +1,98 @@
+(* Author:     Tobias Nipkow
+    Copyright   2000 TUM
+*)
+
+section \<open>Fixed Length Lists\<close>
+
+theory NList
+imports Main
+begin
+
+definition nlists :: "nat \<Rightarrow> 'a set \<Rightarrow> 'a list set"
+  where "nlists n A = {xs. size xs = n \<and> set xs \<subseteq> A}"
+
+lemma nlistsI: "\<lbrakk> size xs = n; set xs \<subseteq> A \<rbrakk> \<Longrightarrow> xs \<in> nlists n A"
+  by (simp add: nlists_def)
+
+lemma nlistsE_length [simp](*rm simp del*): "xs \<in> nlists n A \<Longrightarrow> size xs = n"
+  by (simp add: nlists_def)
+
+lemma less_lengthI: "\<lbrakk> xs \<in> nlists n A; p < n \<rbrakk> \<Longrightarrow> p < size xs"
+by (simp)
+
+lemma nlistsE_set[simp](*rm simp del*): "xs \<in> nlists n A \<Longrightarrow> set xs \<subseteq> A"
+unfolding nlists_def by (simp)
+
+lemma nlists_mono:
+assumes "A \<subseteq> B" shows "nlists n A \<subseteq> nlists n B"
+proof
+  fix xs assume "xs \<in> nlists n A"
+  then obtain size: "size xs = n" and inA: "set xs \<subseteq> A" by simp
+  with assms have "set xs \<subseteq> B" by simp
+  with size show "xs \<in> nlists n B" by(clarsimp intro!: nlistsI)
+qed
+
+lemma nlists_n_0 [simp]: "nlists 0 A = {[]}"
+unfolding nlists_def by (auto)
+
+lemma in_nlists_Suc_iff: "(xs \<in> nlists (Suc n) A) = (\<exists>y\<in>A. \<exists>ys \<in> nlists n A. xs = y#ys)"
+unfolding nlists_def by (cases "xs") auto
+
+lemma Cons_in_nlists_Suc [iff]: "(x#xs \<in> nlists (Suc n) A) \<longleftrightarrow> (x\<in>A \<and> xs \<in> nlists n A)"
+unfolding nlists_def by (auto)
+
+lemma nlists_not_empty: "A\<noteq>{} \<Longrightarrow> \<exists>xs. xs \<in> nlists n A"
+by (induct "n") (auto simp: in_nlists_Suc_iff)
+
+
+lemma nlistsE_nth_in: "\<lbrakk> xs \<in> nlists n A; i < n \<rbrakk> \<Longrightarrow> xs!i \<in> A"
+unfolding nlists_def by (auto)
+
+lemma nlists_Cons_Suc [elim!]:
+  "l#xs \<in> nlists n A \<Longrightarrow> (\<And>n'. n = Suc n' \<Longrightarrow> l \<in> A \<Longrightarrow> xs \<in> nlists n' A \<Longrightarrow> P) \<Longrightarrow> P"
+unfolding nlists_def by (auto)
+
+lemma nlists_appendE [elim!]:
+  "a@b \<in> nlists n A \<Longrightarrow> (\<And>n1 n2. n=n1+n2 \<Longrightarrow> a \<in> nlists n1 A \<Longrightarrow> b \<in> nlists n2 A \<Longrightarrow> P) \<Longrightarrow> P"
+proof -
+  have "\<And>n. a@b \<in> nlists n A \<Longrightarrow> \<exists>n1 n2. n=n1+n2 \<and> a \<in> nlists n1 A \<and> b \<in> nlists n2 A"
+    (is "\<And>n. ?list a n \<Longrightarrow> \<exists>n1 n2. ?P a n n1 n2")
+  proof (induct a)
+    fix n assume "?list [] n"
+    hence "?P [] n 0 n" by simp
+    thus "\<exists>n1 n2. ?P [] n n1 n2" by fast
+  next
+    fix n l ls
+    assume "?list (l#ls) n"
+    then obtain n' where n: "n = Suc n'" "l \<in> A" and n': "ls@b \<in> nlists n' A" by fastforce
+    assume "\<And>n. ls @ b \<in> nlists n A \<Longrightarrow> \<exists>n1 n2. n = n1 + n2 \<and> ls \<in> nlists n1 A \<and> b \<in> nlists n2 A"
+    from this and n' have "\<exists>n1 n2. n' = n1 + n2 \<and> ls \<in> nlists n1 A \<and> b \<in> nlists n2 A" .
+    then obtain n1 n2 where "n' = n1 + n2" "ls \<in> nlists n1 A" "b \<in> nlists n2 A" by fast
+    with n have "?P (l#ls) n (n1+1) n2" by simp
+    thus "\<exists>n1 n2. ?P (l#ls) n n1 n2" by fastforce
+  qed
+  moreover assume "a@b \<in> nlists n A" "\<And>n1 n2. n=n1+n2 \<Longrightarrow> a \<in> nlists n1 A \<Longrightarrow> b \<in> nlists n2 A \<Longrightarrow> P"
+  ultimately show ?thesis by blast
+qed
+
+
+lemma nlists_update_in_list [simp, intro!]:
+  "\<lbrakk> xs \<in> nlists n A; x\<in>A \<rbrakk> \<Longrightarrow> xs[i := x] \<in> nlists n A"
+  by (metis length_list_update nlistsE_length nlistsE_set nlistsI set_update_subsetI)
+
+lemma nlists_appendI [intro?]:
+  "\<lbrakk> a \<in> nlists n A; b \<in> nlists m A \<rbrakk> \<Longrightarrow> a @ b \<in> nlists (n+m) A"
+unfolding nlists_def by (auto)
+
+lemma nlists_append:
+  "xs @ ys \<in> nlists k A \<longleftrightarrow>
+   k = length(xs @ ys) \<and> xs \<in> nlists (length xs) A \<and> ys \<in> nlists (length ys) A"
+unfolding nlists_def by (auto)
+
+lemma nlists_map [simp]: "(map f xs \<in> nlists (size xs) A) = (f ` set xs \<subseteq> A)"
+unfolding nlists_def by (auto)
+
+lemma nlists_replicateI [intro]: "x \<in> A \<Longrightarrow> replicate n x \<in> nlists n A"
+ by (induct n) auto
+
+end