XsltGoodPractice

Individual unsorted comments for now. These couuld be better organized and include more basic suggestions as well at some point.

1. Any changes you write into the XSLT for the purposes of your project should be parameterized in such a way that anyone not using your parameter will see NO CHANGE OF BEHAVIOR at all.
2. If for whatever reason it is not technically possible to parameterize away your changes (as seems to be the case with recent additions of <seg> to gaps and text fields, which caused two separate breakages in existing usage), then YOU MUST warn all active projects (i.e. those who use the live rather than tagged XSLT) before you commit, and ask them to test your changes against their XML.