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[xsl] Newbie Question: Convert "flat" hierarchy to nested hierarchy

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I'm a newbie among newbies in XSLT, so I request your patience.

I'm interested in knowing if I can write an XSLT to transform a "flat"
hierarchy, representing the hierarchy of a document, into a true
nested hierarchy. And do the reverse direction transformation.

To illustrate, here's an example of a "flat" hierarchy, where the
hierachical level of each {markupx} item is specified using the
'lvl' attribute. (Each {markupx} item contains a mix of marked-up
content.)

   <flat lvl="1"> {markupA} </flat>
   <flat lvl="2"> {markupB} </flat>
   <flat lvl="2"> {markupC} </flat>
   <flat lvl="1"> {markupD} </flat>
   <flat lvl="2"> {markupE} </flat>
   <flat lvl="2"> {markupF} </flat>
   <flat lvl="3"> {markupG} </flat>
   <flat lvl="4"> {markupH} </flat>
   <flat lvl="4"> {markupI} </flat>
   <flat lvl="3"> {markupJ} </flat>
   <flat lvl="3"> {markupK} </flat>
   <flat lvl="2"> {markupL} </flat>
   <flat lvl="1"> {markupM} </flat>


Here's what I'd like to transform it into, and back again:

   <nest> {markupA}
      <nest> {markupB} </nest>
      <nest> {markupC} </nest>
   </nest>
   <nest> {markupD}
      <nest> {markupE} </nest>
      <nest> {markupF}
         <nest> {markupG}
            <nest> {markupH} </nest>
            <nest> {markupI} </nest>
         </nest>
         <nest> {markupJ} </nest>
         <nest> {markupK} </nest>
      </nest>
      <nest> {markupL} </nest>
   </nest>
   <nest> {markupM} </nest>


Assume also that there will be other shared attributes for both <flat>
and <nest> -- I've left them out in the above markup examples to make
it easier to visualize the problem.


Are the two-way transformations doable in XSLT?

I humbly request the discussion focus on the feasibility of the
two-way transformation, and not on alternatives to the markup models
I'm working with. I don't have much say in the matter since this task
was dropped in my lap (and having to come up to speed in XSLT.)

Thanks!

Mark
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