Sum and multiply

[roganov]

Hello,

According to docs, sum and multiply is implemented, but I can't get this to work:

func main() {
	doc, err := xmlquery.Parse(strings.NewReader(`
<items>
    <item type="goods">
        <quantity unit="pcs">1</quantity>
        <unit-price currency="PLN">2</unit-price>
        <VAT>7</VAT>
    </item>
    <item type="goods">
        <quantity unit="pcs">10</quantity>
        <unit-price currency="PLN">20</unit-price>
        <VAT>7</VAT>
    </item>
    <item type="goods">
        <quantity unit="pcs">100</quantity>
        <unit-price currency="PLN">200</unit-price>
        <VAT>7</VAT>
    </item>
</items>
`))
	expr, err := xpath.Compile("sum(/items/item/(quantity * unit-price))")
	if err != nil {
		panic(err)
	}
	res := expr.Evaluate(xmlquery.CreateXPathNavigator(doc)).(float64)
	print(res)
}

This code panics with
panic: sum(/items/item/(quantity * unit-price)) has an invalid token