XML : What is the correct HTTP Vary header if content differs for XML accepted/not and gzip accepted/not

I found header("Vary: Accept"); to be the one to demark content that differs based on if the user agent will accept application/xhtml+xml output or just text/html and header("Vary: Accept-Encoding"); if there is a gzipped and a non gzipped version of the page. Now what if there are four versions - gzipped XML / non-gzipped XML / gzipped text/html / non-gzipped text/html - what is the nominally correct header?

I am aware that setting anything but Vary: User-Agent will make IE6 not cache the document and that IE6 won't cache gzipped documents . Also aware that there are IE6 versions that have a bug in the gzip decompression so that they will say they can handle it but can't.

Note: Interest in IE6 is academic.