call-template dynamically

this xsl:
<xsl:variable name="var-attr">
	<xsl:choose test="1">attr_1</xsl:choose>
	<xsl:otherwise>attr_2</xsl:otherwise>
</xsl:variable>
<tag><xsl:attribute name="{$var-attr}"/></tag>

produces:
<tag attr_1=""></tag>

while the following xsl:
<xsl:variable name="var-tmplt">
	<xsl:choose test="1">tmplt_1</xsl:choose>
	<xsl:otherwise>tmplt_2</xsl:otherwise>
</xsl:variable>
<xsl:call-template name="{$var-tmplt}"/>

will produce (using xalan):
XSL Error: Could not find template named: {$var-tmplt}

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