Filtering sequences of atomic types

Dear all,

  I'm trying to filter a sequence using a predicate that involves a node set.
What I actually want to do is something like

$string-sequence[. = //@xml:id]

i.e. get all items of a sequence that occur as an xml:id attribute somewhere
in the document.

As that gives me the error message "XPTY0020: Cannot select a node here: the
context item is an atomic value" (with Saxon 9.1), I tried

$string-sequence[. = (for $n in (//@xml:id) return xs:string($n))]

or even

('foo', 'bar')[. = (for $n in (//@name) return 'foo')]

where the latter should actually return something like ('foo', 'foo', ...),
and ('foo', 'bar')[. = ('foo', 'foo')] works, of course.

My current workaround is translating the string sequence to a sequence of text
nodes, but why doesn't it work with a sequence of atomic types?

Thanks for any help,

Christoph

-- 
Christoph Lange, Jacobs Univ. Bremen, http://kwarc.info/clange, Skype duke4701