sum within template

Hi,

I have a template,

<xsl:template match="purchaseOrder/items/quantity">
<xsl:element name="quant"><xsl:value-of select="current()"/></xsl:element>
</xsl:template>

Now the occurence of items is unbounded.I would like to get the sum of all
the 'quantity' fields within items.

By changing value-of select=sum(current()) it throws an error.Any ideas how
i could get the sum of all 'quantity' without removing template?

Thanks,
Shadab