pairing sibling nodes with pre-post-order

I would like to know if there is a better way to do this with
"pre-post-order"...

In using HtmlPrag and SXML tools to extract information from one Web
page, I need to treat adjacent "td" elements in pairs.

I currently use SXPath to find the "td" elements in a particular part of
the document, then use "pre-post-order" to transform them into the
desired SXML.  The following solution uses a variable to save the first
occurrence in a pair:

    (pre-post-order
     '((td "A1") (td "A2") (td "B1") (td "B2") (td "C1") (td "C2"))
     `((td *preorder* .
           ,(let ((left #f))
              (lambda (tag . elems)
                (if left
                    (begin0 (append (list '*pair) left elems)
                      (set! left #f))
                    (begin (set! left elems)
                           '())))))))
    ==> 
    (() (*pair "A1" "A2") () (*pair "B1" "B2") () (*pair "C1" "C2"))

One non-"pre-post-order" approach would be to use SXPath to pattern-match 
on the children of the "td" elements, and that might be more resilient.

If I were coding functionally in plain Scheme, I could just use pair of
mutually-recursive procedures to effect the alternating behavior.  But I
would like to keep all my code within the SXPath + "pre-post-order"
framework for now.

In any case, I am interested in a general "pre-post-order" solution to
this kind of problem.

-- 
                                             http://www.neilvandyke.org/


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