Lisp question: odd fontification problem

[thorne <thinly-disguised@spam-sucks.foo>]

Hello.  I have been tinkering with setting up a comfortable editing
onvironment (comfortable for me, that is) for editing fiction
manuscripts.  I asked a question about this a while back and got very
useful help, so i now have another, odder question.  I have a function
that is meant to toggle fontification of strings dimlimited by a
character as underlined.  So, in my use, a string with underscores
would be underlined, and the underscores themselves would be
invisible.  So it works.  Horray!  But when i toggle it back off,
something strange happens: the underlining disappears, but the
invisible underscores stay invisible... even if i go to
fundamental-mode, do (font-lock-fontify-buffer) or (and this is what i
don't understand) even if i do (kill-all-local-variables)--the
underscores stay invisible.  I am guessing my whole approach is bad,
but in any case, i am pretty mystified, so here's the code:

(defun toggle-underline-by-font-lock (delimiter)
  "Toggle font-lock-based underlinging of text between DELIMITER.
Simultaneusly make the string DELIMITER itself invisible.

Default delimiter defined by `underline-by-font-lock-delimiter'."
  (interactive "P")
  (let* ((delim (if delimiter
 		    (read-string "Delimiter: ")
		  underline-by-font-lock-delimiter))
	 (delim-regex
	  (concat "\\(" delim "\\)\\([^" delim
		  "]*\\)\\(" delim "\\)\\(.\\)")))
    (funcall (if underline-by-font-lock-flag
		 #'font-lock-remove-keywords
	       #'font-lock-add-keywords)
	     nil
	     `((,delim-regex
		(1 '(face nil invisible t))
		(2 'underline)
		(3 '(face nil invisible t))
		(4 'default))))
    (setq underline-by-font-lock-flag
	  (not underline-by-font))
    (font-lock-fontify-buffer)))

    -- 
þ    theron tlåx    þ
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