How much of the ESI spec is implemented?

[Kurt Mackey]

So ESI looks interesting, but I can't quite figure out the extent to which Varnish supports it.

For instance, the ESI expressions look ridiculously useful in conjunction with the include tag.  Can you do things like this?

<esi:include src="http://example.com/users/$(COOKIE{username})/$(COOKIE{token})"/>

I'm not good enough to completely understand the source, but it looks like that example may not be possible.

It would be awesome to see expression support and support for the choose/when/otherwise tags as well.  Is that the type of thing that someone could sponsor for development?

-Kurt