def out_XML io,indent=nil

      if indent
        Xyml.rawobj2domobj(@root).write(io,indent.to_i)
      else
        sio=StringIO.new
        Xyml.rawobj2domobj(@root).write(sio)
        sio.rewind
        io.print sio.read,"\n"
      end
      io.close
    end
def out_XML io,indent=nil

      if indent
        Xyml.rawobj2domobj(@root).write(io,indent.to_i)
      else
        sio=StringIO.new
        Xyml.rawobj2domobj(@root).write(sio)
        sio.rewind
        io.print sio.read,"\n"
      end
      io.close
    end
save an XML file corresponding to the tree data in the self through the designated IO.

 自身のツリーデータを、指定されたIOを通して、XMLファイルに保存する。
 ==== Args
 _indent_(if not nil) :: a saved XML file is formatted with the designaged indent.
      xyml_tree=Xyml::Document.new({a: [{b: "ccc"},{d: ["eee"]}]})
      #-> [{a: [{b: "ccc"},{d: ["eee"]}]}]
      xyml_tree.out_XML(File.open("aaa.xml","w"))
      #-> aaa.xml
      # <a b="ccc">
      #   <d>eee</d>
      # </a>