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 62 in the spec for more info):
fn:abs
fn:ceiling
fn:floor
fn:round
fn:round-half-to-even (2 versions)

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