<div metal:use-macro="b">
  <div metal:define-macro="a" id="a">
    <metal:block define-slot="slot">default value a</metal:block>
  </div>
  <div metal:define-macro="b" metal:use-macro="a" id="b">
  </div>
  <div metal:fill-slot="slot">main value</div>
</div>