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)