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 60 and 61 in the spec for more info):
fn:true
fn:false
fn:not
I believe we should have a minimum of 5 tests for each function, including negative tests.