                  ********
                 *        *
                 *  TODO  *
                 *        *
                  ********

Lifetime of result Value Tree, make sure we keep pointers. Exslt
handling of node set values is especially nasty in this respect,
lots of potential mem leaks...

Pattern tester:
  -> try to optimize for ID scan and tests.

Pattern scanner:
  -> add error checks on all returns

Sorting:
  -> add lang and case-order
  -> add foreign sorting functions (interfaces ?).

                  ********
                 *        *
                 *  DONE  *
                 *        *
                  ********

Design:
  - should transforms for a given stylesheet be thread clean,
    -> the precompilation now occur only at stylesheet processing
       time (except the binding for named templates and extension
       functions which need to be computed once at run-time).
       Multiple threads should be able to reuse the same stylesheet
       now.

Embedding Stylesheets:
  - exampl                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    