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">
        <geom type="sphere" size=".1"/>
      </body>

      <frame name="frame2" pos=".01 .02 .03">

        <geom name="geom2" type="sphere" size=".1"/>
        <body name="root2">
          <geom type="sphere" size=".1"/>
          <body name="root22">
            <geom type="sphere" size=".1"/>
          </body>
        </body>

        <geom name="geom3" type="sphere" size=".1"/>
        <body name="root3">
          <geom type="sphere" size=".1"/>
          <body name="root5">
            <geom type="sphere" size=".1"/>
          </body>
        </body>
      </frame> <!-- end frame1 -->
    </frame><!-- end frame2 -->
  </worldbody>
</mujoco>