Models of SiSU xml 
[early attempt at modelling, not definitive]
* xsd - xml schema
* rnc - relax ng compact format
* rng - relax ng xml format

* input (markup) formats
  * sax (.sxs.xml)
  * dom (.sxd.xml)
  * node (.sxn.xml)

* output formats
  * sax (sax.xml)
  * dom (dom.xml)