Joints forming a closed loop of linkages - parallel linkages with SDF

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Given the joints have a parent child relationship, is it possible to form loops? If not, are there workarounds? Does this vary between versions of Gazebo?

For context, I have been trying to adapt this urdf ( https://code.google.com/p/utexas-art-ros-pkg/source/browse/branches/sandbox/stacks/artexperimental/artgazebo_plugins/urdf/ackermann.urdf.xacro?r=1545 ) for Gazebo 1.5 that is included with the ROS-groovy. So far I have this ( https://gist.github.com/ftalex/5988718 )

Originally they used player and did this:

  <gazebo>
    <joint:hinge name="ackermann_right_bar_joint">
    <body1>ackermann_bar_link</body1>
    <body2>front_right_bar_link</body2>
    <anchor>ackermann_bar_link</anchor>
    <axis>0 0 1</axis>
    <anchorOffset>0 -0.45 0</anchorOffset>
  </joint:hinge>
</gazebo>

The original has a set of links and with joints so that they form parallel linkages, the two player joints close the loop on each side. I tried to just add joints to replace those in player:

<joint name='ackermann_left_bar_joint' type='revolute'>
        <parent>ackermann_bar_link</parent>
        <child>front_left_bar_link</child>
        <axis>
            <xyz>0.000000 0.000000 1.000000</xyz>
            <limit>
                <lower>-0.500000</lower>
                <upper>0.500000</upper>
                <effort>100.000000</effort>
                <velocity>10.000000</velocity>
            </limit>
            <dynamics/>
        </axis>
</joint>

This seems to do nothing and doesn't give any errors.

Eventually I want to use this with a plugin, but I am still learning about plugins.