How do I use non-literals in <xsl:if statements

I want to compare file_date with another element or attribute, not a literal.  If anyone knows a way to accomplish test statements without using literals, please let me know.  I tried to use variables but my XSL processor must not support them (i.e they're always empty after assignment).  I'm using the LotusXSL processor.

XML INSTANCE:

	<file previous_date="03/13/99">
		<file_date>04/03/99</file_date>
	</file>

XSL STATEMENT (USING LITERAL):

<xsl:if test="file_date[not(. = '03/13/99')]">do something</xsl:if>

XSL STATEMENT (NOT USING A LITERAL)

<xsl:if test="file_date[not(. = @previous_date)]"> do something </xsl:if>



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