class Nokogiri::XML::Document

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