<list>

  <object id="a">

    <object id="a1"/>

    <object id="a2"/>

    <object id="a3"/>

    <object id="a4"/>

  </object>

  <object id="b">

    <object id="b1"/>

    <object id="b2"/>

    <object id="b3"/>

  </object>

<!-- one2one link -->

  <link type="arc" role="mapping" from="a1" to="b1"/>

<!-- grouping -->

  <link type="extended" id="groupa">

    <link type="locator" ref="a1"/>

    <link type="locator" ref="a2"/>

    <link type="locator" ref="a3"/>

  </link>

<!-- grouping -->

  <link type="extended" id="groupb">

    <link type="locator" ref="b1"/>

    <link type="locator" ref="b2"/>

  </link>

<!-- many2many as a 1:1 between groups-->

  <link type="arc" role="mymapping" from="groupa" to="groupb"/>

</list>