procedure main()
    bTree := [1, [2, [4, [7]], [5]], [3, [6, [8], [9]]]]
    mapTree(bTree, write)
    bTree := [1, ["two", ["four", [7]], [5]], [3, ["six", ["eight"], [9]]]]
    mapTree(bTree, write)
end

procedure mapTree(tree, f)
    every f(\tree[1]) | mapTree(!tree[2:0], f)
end