Match tree traverser

sub descend-into(Match $m, :$key = "TOP", :&action, :@skip) {
    action($m, $key);
    for %($m).keys -> $key {
        next if $key eq any @skip;
        given $m{$key} {
            when Match { descend-into($_, :$key, :&action, :@skip) }
            when Array { descend-into($_, :$key, :&action, :@skip) for .list }
            default { die "Unknown thing $_.WHAT() in parse tree!" }
        }
    }
}