Better control of headers on instance src

The src and resource attributes appear to be shorthand for a particular type of submission.

It may make sense to allow a submission attribute on instance to allow a full submission to prepop the instance data. This would allow control over headers, etc.

One could also use some locally stored prepop data in one instance to them help fuel a more exacting submission to get more data related to that prepop data from the server, e.g.

<instance src="file:me.xml"> <name/> </instance>

<instance submission="S"/>

<submission id="S" _get stuff related to me_ />