mujoco

<mujoco model="parent_merge">
  <compiler conflict="merge"/>
  <option timestep="0.005" iterations="50" gravity="0 0 -10"/>
  <asset>
    <model name="child" file="child_mergable.xml"/>
  </asset>
  <worldbody>
    <body name="parent_body">
      <geom name="parent_geom" size="2"/>
      <attach model="child" body="child_body" prefix="child/"/>
    </body>
  </worldbody>
</mujoco>