I am sure this is a hoary old one, but I could not work out the right
runes to achieve it, nor the right keywords to search for to find an
answer

I a have a for-each loop over the W elements. Inside the for-each I
want to select one X with each value of Y that exists within that W.

So for the above example, for //W[1] I want to get the first and third
or second and third X elements, for //W[2] I want all three X elements
and for //W[3] I want any one of the X elements, but only one of them.

I hope that bit is clear.
..
I want a node-set so I can do a for-each on that, so I would prefer to
find a select specifier based on clever use of axes and predicates.

Two answers are below ... both over a node set
... but the first is the variable-based method
which allows one to do grouping directly over
subtrees, but requires an <xsl:if> in the
loop.  The second is using the Muenchian method
and gives a pure node set but the subtree
distinction achieved through the use of the
parent's generated identifier may make those
responsible for maintaining it difficult to understand what is going on.

I am stuck with using XSL 1.0

t:\ftemp>type peter.xml
<V>
 <W>
  <Z/>
      <X Y='0'/>
   <X Y='0'/>
    <X Y='1'/>
  <Z/>
  </W>
 <W>
  <Z/>
     <Z/>
      <X Y='0'/>
    <X Y='1'/>
     <X Y='2'/>
     </W>
  <W>
    <X Y='2'/>
     <X Y='2'/>
      <X Y='2'/>
  <Z/>
     <Z/>
         </W>
</V>

t:\ftemp>type peter.xsl
<?xml version="1.0" encoding="US-ASCII"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
                version="1.0">

<xsl:template match="V">
  <xsl:for-each select="W">
    <xsl:value-of select="concat('/V/W[',position(),']')"/>
    <xsl:text>
</xsl:text>
    <xsl:variable name="Xs" select="X"/>
    <xsl:for-each select="$Xs">
      <xsl:if test="generate-id(.)=generate-id($Xs[@Y=current()/@Y][1])">
        <xsl:value-of select="concat('got X/@Y=&quot;',@Y,'&quot;')"/>
        <xsl:text>
</xsl:text>
      </xsl:if>
    </xsl:for-each>
  </xsl:for-each>
</xsl:template>

</xsl:stylesheet>
t:\ftemp>xslt peter.xml peter.xsl con
/V/W[1]
got X/@Y="0"
got X/@Y="1"
/V/W[2]
got X/@Y="0"
got X/@Y="1"
got X/@Y="2"
/V/W[3]
got X/@Y="2"

t:\ftemp>type peter2.xsl
<?xml version="1.0" encoding="US-ASCII"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
                version="1.0">

<xsl:template match="V">
  <xsl:for-each select="W">
    <xsl:value-of select="concat('/V/W[',position(),']')"/>
    <xsl:text>
</xsl:text>
    <xsl:variable name="Xs" select="X"/>
    <xsl:for-each select="$Xs[generate-id(.)=
            generate-id(key('Xs',concat(generate-id(..),' ',@Y))[1])]">
      <xsl:value-of select="concat('got X/@Y=&quot;',@Y,'&quot;')"/>
      <xsl:text>
</xsl:text>
    </xsl:for-each>
  </xsl:for-each>
</xsl:template>

</xsl:stylesheet>
t:\ftemp>xslt peter.xml peter2.xsl con
/V/W[1]
got X/@Y="0"
got X/@Y="1"
/V/W[2]
got X/@Y="0"
got X/@Y="1"
got X/@Y="2"
/V/W[3]
got X/@Y="2"

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