mujoco

<mujoco model="contact subtree">
  <asset>
    <model file="humanoid.xml"/>
  </asset>

  <worldbody>
    <light pos="0 0 5"/>
    <geom name="floor" type="plane" size="2 2 .01"/>

    <!-- two horizontal humanoids, in the air, one above the other -->
    <frame pos="-.5 0 1.2" euler="0 90 0">
      <attach model="Humanoid" body="torso" prefix="1"/>
    </frame>
    <frame pos="-.5 0 0.5" euler="0 90 -90">
      <attach model="Humanoid" body="torso" prefix="2"/>
    </frame>
  </worldbody>

  <sensor>
    <contact name="all"/>
    <contact name="w_t1" body1="world" subtree2="1torso"/>
    <contact name="w_t2" body1="world" subtree2="2torso"/>
    <contact name="t1" subtree1="1torso"/>
    <contact name="t2" subtree1="2torso"/>
    <contact name="t1_t1" subtree1="1torso" subtree2="1torso"/>
    <contact name="t2_t2" subtree1="2torso" subtree2="2torso"/>
    <contact name="t1_t2" subtree1="1torso" subtree2="2torso"/>
    <contact name="t2_t1" subtree1="2torso" subtree2="1torso"/>
  </sensor>
</mujoco>