daf:trace needs to be usable for complex elements.

E.g., fn:exists(/foo/bar) where bar is a complex element - this works
but fn:exists(daf:trace(/foo/bar, "bar")) - this fails - silently saying doesn't exist, but
actually an error is being masked that /foo/bar is of the wrong type - simple was expected.

Need to change daf:trace to not care if the argument is simple or complex.