[xsl] Selecting from a sequence

Subject: [xsl] Selecting from a sequence
From: "Mailing Lists Mail daktapaal@xxxxxxxxx" <xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx>
Date: Thu, 5 Feb 2015 18:31:40 -0000
Hey All,
I have a logic that says :

if $a found , then use it, else use $b.
I can do this like

<xsl:value-of select = "if(string($a)) then $a else $b"/>

Can I do something like
<xsl:value-of select = "($a,$b)[1]"/>

Not sure , when this will work and when it wont. Or will it work at all..

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