procedure main()
    bTree := [1, [2, [4, [7]], [5]], [3, [6, [8], [9]]]]
    showTree(bTree, preorder|inorder|postorder|levelorder)
end

procedure showTree(tree, f)
    writes(image(f),":\t")
    every writes(" ",f(tree)[1])
    write()
end

procedure preorder(L)
    if \L then suspend L | preorder(L[2|3])
end

procedure inorder(L)
    if \L then suspend inorder(L[2]) | L | inorder(L[3])
end

procedure postorder(L)
    if \L then suspend postorder(L[2|3]) | L
end

procedure levelorder(L)
    if \L then {
        queue := [L]
        while nextnode := get(queue) do {
            every put(queue, \nextnode[2|3])
            suspend nextnode
            }
        }
end