robot ur5 {
  root_link link base_link
  joints {
    joint shoulder_pan_joint {
      parent base_link
      child link shoulder_link
    }
  }	
}

<robot name="ur5">
  <!-- ur5 robot links and joints and more -->
  
  <link name="base_link">
  </link>
  
  <joint name="shoulder_pan_joint">
      <parent link="base_link"/>
      <child link="shoulder_link"/>
  </joint>
  
  <link name="shoulder_link">
  </link>
</robot>