<sdf version='1.6'>
<model name='top_level_model'>
  <link name='parent'/>
  <link name='child'/>
  <model name='nested_model'>
    <link name='nested_link01'/>
  </model>
  <joint name='top_level_joint' type='revolute'>
    <parent>parent</parent>
    <child>child</child>
    <axis>
      <xyz>1 0 0</xyz>
    </axis>
  </joint>
</model>
</sdf>
