class Asciidoctor::HTML5::DocumentTemplate

def self.outline(node, to_depth = 2)
  toc_level = nil
  sections = node.sections
  unless sections.empty?
    toc_level, indent = ''
    nested = true
    unless node.is_a?(Document)
      if node.document.doctype == 'book'
        indent = '    ' * node.level unless node.level == 0
        nested = node.level > 0
      else
        indent = '    ' * node.level
      end
    end
    toc_level << "#{indent}<ol>\n" if nested
    sections.each do |section|
      toc_level << "#{indent}  <li><a href=\"##{section.id}\">#{!section.special && section.level > 0 ? "#{section.sectnum} " : ''}#{section.attr('caption')}#{section.title}</a></li>\n"
      if section.level < to_depth && (child_toc_level = outline(section, to_depth))
        if section.document.doctype != 'book' || section.level > 0
          toc_level << "#{indent}  <li>\n#{child_toc_level}\n#{indent}  </li>\n"
        else
          toc_level << "#{indent}#{child_toc_level}\n"
        end
      end
    end
    toc_level << "#{indent}</ol>" if nested
  end
  toc_level
end