Please excuse the noob kind of question. I've been trying to select a
node into a variable, but I want to change the value of one of the
attributes somewhere within the node. For example, if I have the
following node:

<P id="3">
  <As>
     <a value="4" />
   </As>
  <N value="1">
    <S>
       <sA name="foo" />
    </S>
   <T value="X." />
 </N>
</P>

I'm trying to copy P as-is except I want to change the value attribute
of T to a substring (e.g., substring-before(.,'.')).

So, I'm not sure how to properly construct a variable without
explicitly selecting every element (especially because there could be
any number of elements, but all I want to do is change //T[1]/@value.

I've tried something like:

<xsl:variable name="MyVar">
  <xsl:copy-of select="//P" />
   <xsl:attribute name="value"
     <xsl:value-of select="//T/substring-before(@value,'.')" />
   </xsl:attribute>
</xsl:variable>

but obviously that didn't work. This seems like a very common thing
that people would want to do, but I can't seem to find any example
code that does this.

Sorry for the noob-type question, but I appreciate any help.

--John