Overpass get output structured (with hierarchy, not flat ?)

I suggest

<osm-script>
  <query type="relation">
    <has-kv k="admin_level" v="9"/>
    <bbox-query e="1" n="2" s="3" w="4"/>
  </query>
  <foreach>
    <print />
    <recurse type="relation-way"/>
    <union>
      <item />
      <recurse type="way-node"/>
    </union>
    <print />
  </foreach>
</osm-script>

This prints for each relation first the relation and then the ways and nodes belonging to this relation. Ways and nodes that belong to more than one relation are repeated to appear after each relation where they belong to.

You can even do the same thing with the ways:

<osm-script>
  <query type="relation">
    <has-kv k="admin_level" v="9"/>
    <bbox-query e="1" n="2" s="3" w="4"/>
  </query>
  <foreach>
    <print />
    <recurse type="relation-way"/>
    <foreach>
      <print />
      <recurse type="way-node"/>
      <print />
    </foreach>
    <print />
  </foreach>
</osm-script>

Or let the ways and relations appear after their members:

<osm-script>
  <query type="relation">
    <has-kv k="admin_level" v="9"/>
    <bbox-query e="1" n="2" s="3" w="4"/>
  </query>
  <foreach>
    <recurse type="relation-way" into="ways"/>
    <foreach from="ways" into="way">
      <recurse type="way-node" from="way" into="nodes"/>
      <print from="nodes"/>
      <print from="way"/>
    </foreach>
    <print from="ways"/>
    <print />
  </foreach>
</osm-script>