add (REXML::Document)

    # File src/rexml/document.rb, line 68
                    def add( child )
                            if child.kind_of? XMLDecl
                                    @children.unshift child
                            elsif child.kind_of? DocType
                                    if @children[0].kind_of? XMLDecl
                                            @children[1,0] = child
                                    else
                                            @children.unshift child
                                    end
                                    child.parent = self
                            else
                                    rv = super
                                    raise "attempted adding second root element to document" if @elements.size > 1
                                    rv
                            end
                    end