mujoco

<mujoco model="load save load test model with 2 frames">

  <worldbody>
    <frame name="frame1" pos=".1 .2 .3">

      <geom name="geom1" type="sphere" size=".1"/>
      <body name="root1">
        <joint name="joint1"/>
        <geom type="sphere" size=".1"/>
      </body>

      <body name="root2">
        <frame name="frame2" pos=".01 .02 .03">
          <geom type="sphere" size=".1"/>
          <body name="root22">
            <geom type="sphere" size=".1"/>
          </body>
        </frame><!-- end frame2 -->
      </body>

    </frame> <!-- end frame1 -->
  </worldbody>

</mujoco>