How to redefine keys?

[Colin Paul Adams]

Although I have been using gnus for many years, I have not been able
to rid myself of the habit of always using the r or R keys, even
though I often should be using the f or F keys.

Taking a hint from the FAQ, I tried adding the following to my .gnus
file:

(define-key gnus-summary-mode-map "r" gnus-summary-followup)
(define-key gnus-summary-mode-map "R" gnus-summary-followup-with-original)

but I get any error about the symbol gnus-summary-followup not being
defined.

What's the best way to go about doing this?
-- 
Colin Paul Adams
Preston Lancashire