traversal

member this.TraverseSubtreesLeftToRight =
        let rec loop (tree:'T LazyTree) lowerLevel rightSiblings =
            let tail =
                lazy
                (
                    match rightSiblings with
                            | first::rest -> loop first (List.append (tree.Children.Force()) lowerLevel) rest
                            | [] ->
                                match lowerLevel with
                                        | first::rest -> loop first [] rest
                                        | [] -> Empty
                )
            Cons(this, tail)
        loop this [] []
    member this.TraverseLeftToRight =
        this.TraverseSubtreesLeftToRight
        |> LazyList.map (fun tree -> tree.Item)