getting text nodes of subtree?

Hi all,
how can I get string which contains text nodes of all children of given
element?

for example what will return

<a>
  aaa
  <b>
    bbb
    <c>
      ccc
    </c>
    BBB
  </b>
  AAA
</a>

aaabbbcccBBBAAA

br,

Jakub


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