operator in xsl

hello all,
I have the following xsl codes
			<xsl:if test="SysID[ != '13' or != '14' or !=
'15']">
				<A>
					<xsl:attribute name="href">
						javascript:selectClient('<xsl:value-of
select="../Params/QueryString" />')
					</xsl:attribute>
					<img src='../images/barcode.gif' align="top"
height="15" width="15" BORDER='0' />
				</A>
			</xsl:if>

When i put != for not equal to the page is not
displayed.  Would you all give me a list of operator
in xsl please!  Even if I tried <> but it wasn't
working to me.
Thanks for your help.

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