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 table 63 in the spec for more info):
fn:concat
fn:substring (2 versions)
fn:string-length
fn:upper-case
fn:lower-case
fn:contains
fn:starts-with
fn:ends-with
fn:substring-before
fn:substring-after

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