def count_files(tree_el):

    case = FileTreeVisitor()

    @case(Root)
    def root(tree_el):
        return reduce(lambda a, b: a + count_files(b), tree_el.childs, 0)

    @case(Folder)
    def folder(tree_el):
        return reduce(lambda a, b: a + count_files(b), tree_el.childs, 0)

    @case(File)
    def file(tree_el):
        return 1

    return case.apply(tree_el)