--- a/List.thy	Wed Feb 08 14:06:37 1995 +0100
+++ b/List.thy	Mon Feb 13 15:12:08 1995 +0100
@@ -23,6 +23,7 @@
   filter    :: "['a => bool, 'a list] => 'a list"
   foldl     :: "[['b,'a] => 'b, 'b, 'a list] => 'b"
   length    :: "'a list => nat"
+  flat      :: "'a list list => 'a list"
   nth       :: "[nat, 'a list] => 'a"
 
 syntax
@@ -74,6 +75,9 @@
 primrec length list
   length_Nil  "length([]) = 0"
   length_Cons "length(x#xs) = Suc(length(xs))"
+primrec flat list
+  flat_Nil  "flat([]) = []"
+  flat_Cons "flat(x#xs) = x @ flat(xs)"
 defs
   nth_def "nth(n) == nat_rec(n, hd, %m r xs. r(tl(xs)))"
 end