mujoco

<mujoco>
  <asset>
    <model name="other" file="child.xml"/>
  </asset>
  <worldbody>
    <!-- attach body to body -->
    <body name="parent">
      <geom name="geom" size="1" pos="0 0 2"/>
      <attach model="other" body="body" prefix="other1"/>
    </body>

    <!-- attach frame to body (world) -->
    <attach model="other" frame="frame" prefix="other2"/>

    <!-- attach frame to frame -->
    <frame name="top" pos="0 0 4">
      <attach model="other" frame="frame" prefix="other3"/>
    </frame>

    <!-- attach body to frame -->
    <frame name="bottom-right" pos="2 0 -1">
      <attach model="other" body="body" prefix="other4"/>
    </frame>
  </worldbody>
</mujoco>