<span metal:define-macro="INNER">
  <span metal:define-slot="INNERSLOT">INNERSLOT</span>
</span>

<xxx metal:use-macro="INNER">
  <xxx metal:fill-slot="INNERSLOT">inner-argument</xxx>
</xxx>

<div metal:define-macro="OUTER">
<div metal:use-macro="INNER">
  <xxx metal:define-slot="OUTERSLOT" metal:fill-slot="INNERSLOT">
    OUTERSLOT
  </xxx>
</div>
</div>

<div metal:use-macro="OUTER">
<span>
  <xxx>
    <div metal:fill-slot="OUTERSLOT">outer-argument</div>
  </xxx>
</span>
</div>

<div metal:use-macro="OUTER">
</div>