# File lib/nokogiri/xml/document.rb, line 211
      def add_child node_or_tags
        raise "Document already has a root node" if root
        node_or_tags = coerce(node_or_tags)
        if node_or_tags.is_a?(XML::NodeSet)
          raise "Document cannot have multiple root nodes" if node_or_tags.size > 1
          super(node_or_tags.first)
        else
          super
        end
      end