Generic tree folder in Perl 6

tree-visit.pl

sub visit($root, &children, :&pre, :&post, :&assemble = -> *%{ Nil }) {
    my ($pre, $post, @children);
    $pre = pre($root) if defined ⪯
    @children = children($root).map: {visit $_, &children, :&pre, :&post, :&assemble};
    $post = post($root, :@children) if defined &post;
    return assemble(:$pre, :$post, :@children, :node($root));
}

=begin pod

=head1 visit

    sub visit($root, &children; :&pre, :&post, :&assemble = -> *% { Nil })

Visits C<$root> and, recursively, all of its children, as returned from
C<children($root)>. Runs C<pre> before and C<post> after visiting the children.

If an named argument was passed for C<assemble>, that routine is called with
named arguments C<node>, C<children>, C<pre> and C<post>, and its return value
is returned from sub C<visit>.

=end pod

say visit($=pod[0], { .content if .isa(Pod::Block) }, :pre(*.Str),
    :assemble(-> :$pre, :@children, *% { join ' ', $pre, @children; }));