
* Optimize, with the last refactoring we lost around 15% of speed because
  one optimization was removed. This optimization consisted to build the
  tree only with the elements that contained an stl command. We have to
  figure out a way to get it back, in the new handler world.

* Cast, right now the content and attributes stl commands only expect strings,
  maybe non string objects should be casted through the str function.

* Comments, now stl code within XML comments is not processed. I guess to
  do this we should relax the way xml is interpreted, maybe we couldn't use
  expat anymore.

