> You can just use XPath here, no need for recursion:
> 
> <xsl:template match="point">
>  <xsl:copy>
>    <xsl:copy-of select="@*"/>
>    <xsl:attribute name="y2">
>      <xsl:value-of 
> select="sum(./@y1|preceding-sibling::point[@x = current()/@x]/@y1)"/>
>    </xsl:attribute>
>  </xsl:copy>
> </xsl:template>
> 

Though that's O(n^2) - a recursive solution might be O(n).

Michael Kay
http://www.saxonica.com/