Lookup efficiency in XALAN?

I have to do some validation in my xsl. 

I'm doing the following type statement 20,000 times, and the lookup file is
around 50,000  lines of  id's <id key="XXYYZZJ"/>.

<xsl:when test="not(document('../master/ids-master.xml')/ids/id[@key=$id])">

It works for a while, then dies with an out of memory error, and it sure is
slow!


Any ideas for a faster/reliable search?


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