diff -r f51d4a302962 -r 5386df44a037 src/Doc/Tutorial/Misc/Tree2.thy
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/Doc/Tutorial/Misc/Tree2.thy	Tue Aug 28 18:57:32 2012 +0200
@@ -0,0 +1,26 @@
+(*<*)
+theory Tree2 imports Tree begin
+(*>*)
+
+text{*\noindent In Exercise~\ref{ex:Tree} we defined a function
+@{term"flatten"} from trees to lists. The straightforward version of
+@{term"flatten"} is based on @{text"@"} and is thus, like @{term"rev"},
+quadratic. A linear time version of @{term"flatten"} again reqires an extra
+argument, the accumulator. Define *}
+(*<*)primrec(*>*)flatten2 :: "'a tree \<Rightarrow> 'a list \<Rightarrow> 'a list"(*<*)where
+"flatten2 Tip xs = xs" |
+"flatten2 (Node l x r) xs = flatten2 l (x#(flatten2 r xs))"
+(*>*)
+
+text{*\noindent and prove*}
+(*<*)
+lemma [simp]: "!xs. flatten2 t xs = flatten t @ xs"
+apply(induct_tac t)
+by(auto);
+(*>*)
+lemma "flatten2 t [] = flatten t"
+(*<*)
+by(simp)
+
+end
+(*>*)