mujoco

<mujoco>
  <option>
    <flag contact="disable"/>
  </option>

  <default>
    <default class="static">
      <geom size=".5 .1 .5" rgba=".5 .7 .5 .3"/>
    </default>
    <default class="free">
      <geom type="box" size=".2" fromto="0 0 0 0 -1 0" rgba=".4 .7 .6 .3"/>
    </default>
  </default>

  <worldbody>
    <geom pos="0 0 -2" type="plane" size="10 10 .01"/>
    <light pos="0 0 20"/>

    <body name="box1" pos="-3 0 0">
      <geom type="box" class="static"/>
    </body>
    <body name="beam1" pos="-3 0 0">
      <freejoint/>
      <geom class="free"/>
    </body>

    <body name="box2" pos="-1 0 0">
      <geom type="box" class="static"/>
    </body>
    <body name="beam2" pos="-1 0 0">
      <freejoint/>
      <geom class="free"/>
    </body>

    <body name="box3" pos="1 0 0">
      <geom type="box" class="static"/>
      <site name="box3" pos=".5 -.1 .5" type="box" size=".05 .05 .05"/>
    </body>
    <body name="beam3" pos="1 0 0">
      <freejoint/>
      <geom class="free"/>
      <site name="beam3" pos=".2 -.1 .2" type="box" size=".05 .05 .05"/>
    </body>

  </worldbody>

  <equality>
    <connect body1="beam1" anchor="0 0 0"/>
    <connect body1="box2" body2="beam2" anchor=".5 .1 .5"/>
    <connect site1="box3" site2="beam3"/>
  </equality>
</mujoco>