The final example in section 3.2.1 Filter Expressions is:

o The following example also illustrates the use of a filter expression as a step in a path expression. It returns the element node within the specified document whose ID value is tiger:

fn:doc("zoo.xml")/fn:id('tiger')


This was a filter expression in XQuery 1.0, when we defined a filter expression  as a primary expression followed by zero or more predicates. We now define a filter expression as an expression followed by a predicate.