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

<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>

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 have run out of ideas now, so I have finally admitted defeat (a hard
thing for an engineer) and am asking for your collective wisdom.

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.