Boolean XPath Expression and sum

Hello List,
I was wondering if anyone knew a way to either
1. Include a test for content within a template match statement, something
like:
<xsl:template match="abuncha/thing !='N/A'">
OR
2. Include a test for content within sum() , something like:
<xsl:value-of select="sum(//thing !='N/A')"/>


I need to sum up the all the values of things where their value is not equal
to N/A, and am having a bit of trouble.
XML might look like
<abuncha>
<thing>1</thing>
<thing>2</thing>
<thing>3</thing>
<thing>5</thing>
<thing>N/A</thing>
</abuncha>

Thanks,
Alex


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