data Tree = Empty | Node( value, left, right )

def
  preorder( Empty )          =  []
  preorder( Node(v, l, r) )  =  [v] + preorder( l ) + preorder( r )

  inorder( Empty )           =  []
  inorder( Node(v, l, r) )   =  inorder( l ) + [v] + inorder( r )

  postorder( Empty )         =  []
  postorder( Node(v, l, r) ) =  postorder( l ) + postorder( r ) + [v]

  levelorder( x ) =
    def
      order( [] )                 =  []
      order( Empty         : xs ) =  order( xs )
      order( Node(v, l, r) : xs ) =  v : order( xs + [l, r] )

    order( [x] )

tree =    Node( 1,
            Node( 2,
              Node( 4,
                Node( 7, Empty, Empty ),
                Empty ),
              Node( 5, Empty, Empty ) ),
            Node( 3,
              Node( 6,
                Node( 8, Empty, Empty ),
                Node( 9, Empty, Empty ) ),
              Empty ) )

println( preorder(tree) )
println( inorder(tree) )
println( postorder(tree) )
println( levelorder(tree) )