XSLT: Selecting nodes based on a group of other nodes

["Mark Sztainbok" <sz@xxxxxxxxxxxxxx>]

Is there a way to select nodes out of a document if they have a value which
matches a value in another group of nodes?

For example,

if I have a document which looks like this:

<RESPONSE>
    <PRODUCT>
        <ID>ABC</ID>
        <NAME>Brown dog</NAME>
    </PRODUCT>
    <PRODUCT>
        <ID>DEF</ID>
        <NAME>Black cat</NAME>
    </PRODUCT>
    <PRODUCT>
        <ID>GHI</ID>
        <NAME>Yellow giraffe</NAME>
    </PRODUCT>
    <PRODUCT>
        <ID>JKL</ID>
        <NAME>Orange monkey</NAME>
    </PRODUCT>
    <SELECTEDPRODUCT>ABC</SELECTEDPRODUCT>
    <SELECTEDPRODUCT>GHI</SELECTEDPRODUCT>
</RESPONSE>

I would like to get a node list of the products which have an ID which is in
a SELECTEDPRODUCT element i.e. products ABC and GHI (Brown dog and Yellow
giraffe) and store it in a variable so I can use it later with for-each

I have tried the following XSLT code but it doesn't work as it only gets the
first product:
<xsl:variable name="IDS" select="/RESPONSE/SELECTEDPRODUCT"/>
<xsl:variable name="products" select="/RESPONSE/PRODUCT[ID = $IDS]/>

and have also tried this, which also did not work:
<xsl:variable name="products"/>
<xsl:for-each select="RESPONSE/SELECTEDPRODUCT">
    <xsl:variable name="product_id" select="text()"/>
    <xsl:choose>
        <xsl:when test="$products">
            <xsl:variable name="products"
select="$products|/RESPONSE/PRODUCT[ID=$product_id])"/>
        </xsl:when>
        <xsl:otherwise>
            <xsl:variable name="products"
select="/RESPONSE/PRODUCT[ID=$product_id]"/>
        </xsl:otherwise>
   </xsl:choose>
</xsl:for-each>

What is the correct way to do this?

Thanks,

Mark


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