How to get parent node with xpath?

Hi!

I'm a bit confused about the way xpath works. Why doesn't it return parent node 
on path ".."? Suppose the following code:

(define (xpath-first path xml)
  (let ((result ((txpath path) xml)))
    (if (pair? result)
        (car result)
        #f)))

(define parent (sxml:add-parents '(parent 
                                   (child "a"))))

Next expression returns 'parent' node as supposed:
(xpath-first "//child/.." parent)

Next two expressions seem to return the same result, but they do not:
(xpath-first ".." (xpath-first "//child" parent))
(xpath-first "parent::*" (xpath-first "//child" parent))

At the same time it is possible to get parent node manually:
((cadr (sxml:aux-node (xpath-first "//child" parent) '*PARENT*)))


So, why doesn't path ".." works? Probably, I should use some other path? 


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