Almost certainly you would be best doing this in two passes,
first make a transformation that _creates_ a stylesheet using the
template patterns generated from the source, then run that generated
stylesheet.

For simple cases, if you must do it in one pass, then something like

<xsl:template match="*">
<xsl:if test="document('start.xml')/START/*[name()=name(current())]">
...

David

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