sub a_callback {
        my ($filter,$element)  = @_;
        # where $element = {
        #          tag      => 'a',
        #          attr     => { href => '/index.html' },
        #          children => [
        #                        XML::LibXML::Text -->    'Go to ',
        #                        XML::LibXML::Element --> 'b'
        #                           with child Text -->   'Home',
        #                        XML::LibXML::Text -->    ' page',
        #                      ],
        #       }
        return 1;
    }