how to completely redefine effects of define-derived-mode

[Xah Lee]

i'm experimenting and learing the use of define-derived-mode to define
various styled comment syntax, and i don't want to restart emacs each
time. Is there a way to do it?

for example, i evaluate:

(define-derived-mode xx-mode fundamental-mode
"xx"
"xx-mode does this and that."

  ;; bash style comment: “# ...”
  (modify-syntax-entry ?# "< b" xx-mode-syntax-table)
  (modify-syntax-entry ?\n "> b" xx-mode-syntax-table)

;;   ;; c style comment “// comment”
;;   (modify-syntax-entry ?\/ ". 12b" xx-mode-syntax-table)
;;   (modify-syntax-entry ?\n "> b" xx-mode-syntax-table)

)

and after i tested it, i want to comment out the perl styled syntax
entry i made, and try a different style such as “// ...” or “/* ...
*/” or “(* ... *)” style.

However, it won't work because the char “#” already got a defined as
comment start. (it won't go away unless i explicitly redefine that
char. To redefine them is somewhat more work than restarting emacs)

I tried to add (unintern xx-mode-syntax-table) before the define-
derived-mode, and eval the buffer again, but that doesn't work. I also
tried (setq xx-mode-syntax-table nil) that also didn't work. I tried
(setq xx-mode nil) in the beginning but that didn't work neither.

is there a way that i can re-eval this and try different settings of
syntax table? (as opposed to restarting emacs each time)

  Xah
∑ http://xahlee.org/

☄