This task is currently delegated to Matteo, the issue is for reference:

Currently, there are testcases for the postfix lookup in our tests for the xquery parser, but there are no testcases for the unary lookup yet. We should add some testcases analogous to those ones, both for the parallelized case (xml-files) and the local execution (doc)

How the lookups should behave can be found in the spec here