in jekyll/plugins/toc_builder_hook.rb [50:63]
def extract_from_node(node, parent_id)
items = []
case node.type
when :ul
items.concat(extract_items(node, parent_id))
when :header
items.push(extract_header(node)) if node.options[:level] == 2
end
return items.length > 0 ? items : nil
end