Truncating output of a node

I am trying to output the first n sentences of a node. I have tried using
for-each with a conditional to stop output but have had no luck.

Given the following XML fragment, what is the best way to output only the
first n sentences? Note that the node has both text and child nodes.

<summary>
    It is best to start a new <span class="highlight">message</span> for a
new thread.  Do not start a
    new thread by replying to an unrelated <span
class="highlight">message</span> and just changing the
    subject line, since the header of your <span
class="highlight">message</span> will contain references
    to the previous <span class="highlight">message</span> and your new
<span class="highlight">message</span>
  will appear in the archive as one of the replies to the original <span
class="highlight">message</span>.
</summary>

Thanks in advance,
Jim


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