prefix arg when mouse clicking on header

[Roland Winkler]

I have some difficulties with the command proced-sort-header in
proced.el. This command is called when the mouse is on the header
line. The first arg of this command is the mouse event. I'd like to
give the command an optional prefix arg. But somehow it seems to me
that clicking on the header line is different from clicking
elsewhere. What is the proper way to access the prefix arg when
clicking on the header line?

I used to have (interactive "e\nP"), but that ignores the prefix arg.

Instead of "P", I really would like to have something like

  (interactive (list last-input-event (or current-prefix-arg 'no-arg)))

(The 'no-arg is similar to 'toggle in define-derived-mode.) 
But again, this ignores the prefix arg. By trial an error I have
found that

  (interactive (list last-input-event (or last-prefix-arg 'no-arg)))

seems to work. But is this the right thing? Why last-prefix-arg
instead of current-prefix-arg? Certainly, this is consistent with
the code letter P being ignored by interactive.

Also I was wondering what is the proper replacement for the code
letter e used by interactive when the list is constructed by hand.
last-input-event is used in commands like Buffer-menu-sort-by-column.
Is last-input-event always equivalent to the code letter e?
If yes, that could go into the elisp manual. If not, what would be
the proper replacement?

Thanks!

Roland