custom &map on a Tree in Perl 6

class Tree {
    has $.node;
    has @.children;
    method new($node, *@children) { self.bless(*, :$node, :@children) }
    method Str {
        @.children ?? "$.node: [@.children.join(", ")]" !! $.node.Str
    }
}

multi map(&c, Tree $t) {
    Tree.new(c($t.node), map { map(&c, $_) }, $t.children)
}

my $t = Tree.new("H2O", Tree.new("H"), Tree.new("O"), Tree.new("O"));
say $t;
say map { .lc }, $t