We have some test coverage for XPath functions already. However, we were originally working on the assumption that XPath itself has already been well-tested. Now that we have our own implementation (DPath), we can no longer make those assumptions.

We need to fully exercise the following string-related functions (See tables 65-67 in the spec for more info):
fn:empty
fn:exists
fn:exactly-one
fn:count
fn:local-name (2 versions)
fn:namespace-uri (2 versions)
fn:nilled

I believe we should have a minimum of 5 tests for each function, including negative tests.