PAQ: further revision of simple HTTP interface for discovery (ISSUE 53)

Ref. http://dvcs.w3.org/hg/prov/raw-file/tip/paq/provenance-access.html

I've made some further changes (since today's telecon) to the proposal for a 
simple HTTP interface for provenance discovery, relating to 
http://www.w3.org/2011/prov/track/issues/53

In particular, I've made a concrete proposal to allow an additional query 
parameter to select the desired result type.

(The practical motivation for this is for AJAX clients in some browsers: the 
interaction between AJAX-requested Accept: headers and browser-supplied headers 
varies between browsers.  In any case, it's often easier in Javascript to just 
request a URI, without having to set extra HTTP headers.)

#g
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