Sorting based on string length in an element

Hello XSLT experts,

I have the following kind of content:

<bookshelf>
  <book><desc>description AAAA</desc></book>
  <book><desc>description AA</desc></book>
  <book><desc>description AAA</desc></book>
  <book><desc>description A</desc></book>
</bookshelf>

I am looking for one stylesheet that sorts the booklist in descending order
depending on the length of the <desc> element. The result would look like
this:

<bookshelf>
  <book><desc>description AAAA</desc></book>
  <book><desc>description AAA</desc></book>
  <book><desc>description AA</desc></book>
  <book><desc>description A</desc></book>
</bookshelf>

Also I would like to have a slight modification of that which cuts the list
at a certain position (e.g. pos = 2) which would give me this result:

<bookshelf>
  <book><desc>description AAAA</desc></book>
  <book><desc>description AAA</desc></book>
</bookshelf>

How would this modified stylesheet look like assuming that it should also
work properly in case the list is shorter than pos?

Kind Regards,
Karl

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