<q:query xmlns:q="http://www.w3.org/2001/06/xqueryx">
  <q:flwr>
    <q:forAssignment variable="$p">
      <q:function name="distinct-values">
        <q:step axis="SLASHSLASH">
          <q:function name="document">
            <q:constant datatype="CHARSTRING">bib.xml</q:constant>
          </q:function>
          <q:identifier>publisher</q:identifier>
        </q:step>
      </q:function>
    </q:forAssignment>
    <q:letAssignment variable="$a">
      <q:function name="avg">
        <q:step axis="CHILD">
          <q:function name="document">
            <q:constant datatype="CHARSTRING">bib.xml</q:constant>
          </q:function>
          <q:step axis="CHILD">
            <q:predicatedExpr>
              <q:identifier>book</q:identifier>
              <q:predicate>
                <q:function name="EQUALS">
                  <q:identifier>publisher</q:identifier>
                  <q:variable>$p</q:variable>
                </q:function>
              </q:predicate>
            </q:predicatedExpr>
            <q:identifier>price</q:identifier>
          </q:step>
        </q:step>
      </q:function>
    </q:letAssignment>
    <q:return>
      <q:elementConstructor>
        <q:tagName>
          <q:identifier>publisher</q:identifier>
        </q:tagName>
        <q:elementConstructor>
          <q:tagName>
            <q:identifier>name</q:identifier>
          </q:tagName>
          <q:step axis="CHILD">
            <q:variable>$p</q:variable>
            <q:nodeKindTest kind="TEXT" />
          </q:step>
        </q:elementConstructor>
        <q:elementConstructor>
          <q:tagName>
            <q:identifier>avgprice</q:identifier>
          </q:tagName>
          <q:variable>$a</q:variable>
        </q:elementConstructor>
      </q:elementConstructor>
    </q:return>
  </q:flwr>
</q:query>