我可以使用XPath函数和XPathNavigator吗?

I've got an XPathNavigator at the root of a document. Several levels down, there's a group of numeric values that I want to sum. I could always loop through the nodes and add them myself, but since I knew the XPath spec included a sum function, I decided to try to use that. I'm running into an error.I've got an XPathNavigator at the root of a doc