###
### walk_html_userparam
###

## Chapter 2 section 2.1

sub walk_html {
  my ($html, $textfunc, $elementfunc, $userparam) = @_;
  return $textfunc->($html, $userparam) unless ref $html;

  my ($item, @results);
  for $item (@{$html->{_content}}) {
    push @results, walk_html($item, $textfunc, $elementfunc, $userparam);
  }
  return $elementfunc->($html, $userparam, @results);
}