urdf to xacro

edit

I'm building my first robot and I'm not understanding XACRO.

I did the robot in urdf and now I'm changing the file to xacro. But I get a lot or errors.

Expat parsing error.  Check that:
 - Your XML is correctly formed
 - You have the xacro xmlns declaration: xmlns:xacro="http://www.ros.org/wiki/xacro"

Traceback (most recent call last):
  File "/opt/ros/indigo/share/xacro/xacro.py", line 60, in <module>
    xacro.main()
  File "/opt/ros/indigo/lib/python2.7/dist-packages/xacro/__init__.py", line 672, in main
    doc = parse(f)
  File "/usr/lib/python2.7/xml/dom/minidom.py", line 1918, in parse
    return expatbuilder.parse(file)
  File "/usr/lib/python2.7/xml/dom/expatbuilder.py", line 928, in parse
    result = builder.parseFile(file)
  File "/usr/lib/python2.7/xml/dom/expatbuilder.py", line 211, in parseFile
    parser.Parse("", True)
xml.parsers.expat.ExpatError: unclosed token: line 1, column 0

I only changed the first lines of the original urdf file that was successfully parsed. I just added this code.

<?xml version="1.0">
<robot xmlns:xacro="http://www.ros.org/wiki/xacro" name="r2d2" >

<xacro:property name="raioRoda" value="0.05"/>