RE: how to prune subtrees in an XQuery result

"where $A/$matches" is not wrong, but given that $A is a singleton, it's
trivially the same as "where $matches".

I think you can still avoid repeating the condition even when the "or"
becomes an "and".

<Results> {
   for $A in doc("data.xml")/ASet/A
   let $matchesB := $A/B[. = ("red","yellow")]
   let $matchesC := $A/C[. = ("chair","table")]   
   where ($matchesB and $matchesC)
     return
      <A> 
       {$A/@*, $matchesB, $matchesC }
      </A>        
}</Results>