Once again on extra headers

[Harry Putnam <reader@newsguy.com>]

I've used the cool extra header code for sometime to generate NOV  and
display a "Keywords: " header like this:

     (setq gnus-extra-headers
           '(To Newsgroups Keywords))
     (setq nnmail-extra-headers gnus-extra-headers)

Now I want to use this same technique to generate NOV and display a
custom header `X-HP-Keywords: '.

Using the same code:

     (setq gnus-extra-headers
           '(To Newsgroups X-HP-Keywords))
     (setq nnmail-extra-headers gnus-extra-headers)

And regenerating NOV in that group isn't doing what I expected.  No
X-HP-Keyword header is displayed.  It isn't clear in the manual
section that covers this if this only works with certain headers or if
one can make it work on a custom header too.

There is language there that says:

  2. The value of these extra headers can be accessed via the
     `gnus-extra-header' function.  Here's a format line spec that will
     access the `X-Newsreader' header:

          "%~(form (gnus-extra-header 'X-Newsreader))@"

But I haven't had to do that for the `Keywords' to work.  Is it my
non-regulation header causing the problem or something else.