(*<*)

 theory Tree2 = Tree:

 (*>*)

 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: *}

 consts flatten2 :: "'a tree => 'a list => 'a list"

 (*<*)

 primrec

 "flatten2 Tip xs = xs"

 "flatten2 (Node l x r) xs = flatten2 l (x#(flatten2 r xs))"

 (*>*)

 text{*\noindent Define @{term"flatten2"} 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

 (*>*)