XML::XSLT

I'm trying to do the following:

XML:
<StepIndex>0</StepIndex>

XSL:
<xsl:template match="StepIndex">
<xsl:value-of select=". + 1"/>
</xsl:template>

How difficult is it to implement basic expressions
like this? I may be able to take a crack at it, but
I'd like a recommendation as to where to start.