class Nokogiri::XML::Document

def add_child(node_or_tags)
  raise "A document may not have multiple root nodes." if (root && root.name != "nokogiri_text_wrapper") && !(node_or_tags.comment? || node_or_tags.processing_instruction?)
  node_or_tags = coerce(node_or_tags)
  if node_or_tags.is_a?(XML::NodeSet)
    raise "A document may not have multiple root nodes." if node_or_tags.size > 1
    super(node_or_tags.first)
  else
    super
  end
end