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(tree, tail)
       loop this [] []
   member this.TraverseLeftToRight =
       this.TraverseSubtreesLeftToRight
       |> LazyList.map (fun tree -> tree.Item)