constant VALUE = 1, LEFT = 2, RIGHT = 3

constant tree = {1, {2, {4, {7, 0, 0}, 0},
                        {5, 0, 0}},
                    {3, {6, {8, 0, 0},
                            {9, 0, 0}},
                        0}}

procedure preorder(object tree)
    if sequence(tree) then
        printf(1,"%d ",{tree[VALUE]})
        preorder(tree[LEFT])
        preorder(tree[RIGHT])
    end if
end procedure

procedure inorder(object tree)
    if sequence(tree) then
        inorder(tree[LEFT])
        printf(1,"%d ",{tree[VALUE]})
        inorder(tree[RIGHT])
    end if
end procedure

procedure postorder(object tree)
    if sequence(tree) then
        postorder(tree[LEFT])
        postorder(tree[RIGHT])
        printf(1,"%d ",{tree[VALUE]})
    end if
end procedure

procedure level_order(object tree, sequence more = {})
    if sequence(tree) then
        more &= {tree[LEFT],tree[RIGHT]}
        printf(1,"%d ",{tree[VALUE]})
    end if
    if length(more) > 0 then
        level_order(more[1],more[2..$])
    end if
end procedure

puts(1,"\n preorder:    ")  preorder(tree)
puts(1,"\n inorder:     ")  inorder(tree)
puts(1,"\n postorder:   ")  postorder(tree)
puts(1,"\n level-order: ")  level_order(tree)