How do you test for "if something exists" using <xsl:if>

How do I express "if x exists"

ie.  if the value of the parameter "x" exists

<xsl:if test="{$x}">
	....	
</xsl:if>

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and what about multiple tests?

ie if the value of "x" exists && the value of "y" exists && the value of "z"
exists

<xsl:if test="{$x && $y && $z}">
	....	
</xsl:if>


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