'To' and 'Newsgroups' extra headers

[romeomedina]

In order to have the 'To' header displayed in the summary buffer,
I put in ~/.gnus.el the following:

(setq gnus-extra-headers '(To))
(setq gnus-summary-line-format
 ":%U%R%z%I %-15,15a| %-20,20~(form (gnus-extra-header 'To))@| %s
%-85=|%4L |%D \n")

. Now, I want gnus to display the 'To' header when it exists, i.e. for
mails,
but the 'Newsgroups' header in its place when the 'To' header does not
exist
and there is the 'Newsgroup' header, i.e. with newsgroups messages.

Any suggestion about the right syntax in ~/.gnus.el?

Thanks,
Rodolfo