abstracted a higher level 'main' XSL
note that 'SingleChoice' is a fake problem type, just a trivial variant of 'MultipleChoice' to test that the include was working -- it can be discarded
here is an example of a fully exported (toy) course, for reference only
fixed the ' disable-output-escaping="yes"' which was omitted
no message
added a template to insert a comment in the output file
changed the choice ID value to include the problem id to make it more reliably unique (looking forward to targeted feedback linkage being added)