sub traversal {
  my $self = shift;
    my @nodes;
    # self is FIRST
    @nodes = ( $self );
    for (@{ $self->{children} }) {
        push @nodes, $_->traversal( );
    }
    return @nodes;
}

##</code><code>##

sub traversal {
  my $self = shift;
    my @nodes;
    # self is FIRST
    @nodes = ( $self );
    push @nodes, @{ $self->{children} };
    $_->traversal() for @{ $self->{children} };
    return @nodes;
}