获取另一个节点的子节点，给定节点名

I have an XML like this:

我有一个这样的XML:

<documentslist>
  <document>
    <docnumber>1</docnumber>
    <docname>Declaration of Human Rights</docname>
    <aoo>lib</aoo>
  </document>
  <document>
    <docnumber>2</docnumber>
    <docname>Fair trade</docname>
    <aoo>lib</aoo>
  </document>
  <document>
    <docnumber>3</docnumber>
    <docname>The wars for water</docname>
    <aoo>lib</aoo>
  </document>
  <!-- etc. -->
</documentslist>
<docum