class MyFormat is CORE::GrammarVisitor
{
  my $fh is public;

  method date ($node) {
    $fh.printf("%04d-%02d-%02d", $node ^. qw(year month day))
  }
}

$0.traverse(MyFormat.new(fh=>$*STDOUT)); # default ctor?

##</code><code>##

  my $sub = sub {print $value};
  $sub.MY{'$value'} = 42;
  $sub();