{def walk

 {def walk.r
  {lambda {:o :t}
   {if {W.equal? :t nil}
    then
    else {if {W.equal? :o preorder}  then {A.get 0 :t} else}
          {walk.r :order {A.get 1 :t}}
         {if {W.equal? :o inorder}   then {A.get 0 :t} else}
          {walk.r :order {A.get 2 :t}}
         {if {W.equal? :o postorder} then {A.get 0 :t} else} }}}

 {lambda {:o :t}
  {S.replace \s by space in {walk.r :o :t}}}}

{def sort
 {lambda {:o :t} {S.sort :o {walk preorder :t}}}}

{def T
 {A.new 1
  {A.new 2
   {A.new 4
    {A.new 7 nil nil} nil}
     {A.new 5 nil nil}}
      {A.new 3
       {A.new 6
        {A.new 8 nil nil}
         {A.new 9 nil nil}} nil}}}

{walk preorder {T}}  -> 1 2 4 7 5 3 6 8 9
{walk inorder {T}}   -> 7 4 2 5 1 8 6 9 3
{walk postorder {T}} -> 7 4 5 2 8 9 6 3 1
{sort < {T}}         -> 1 2 3 4 5 6 7 8 9
{sort > {T}}         -> 9 8 7 6 5 4 3 2 1