my-toggle-bool.el

(defun my-toggle-bool()
  "Invert bool value at point."
  (interactive)
  (skip-chars-backward "YESNOTRUEFALSEtruefalsetnil")
  (or (looking-at "\\(TRUE\\|FALSE\\|true\\|false\\|YES\\|NO\\|t\\|nil\\)")
	  (error "No bool at point"))
  (cond
   ((equal (match-string 0) "TRUE")
	(replace-match "FALSE"))
   ((equal (match-string 0) "FALSE")
	(replace-match "TRUE"))
   ((equal (match-string 0) "true")
	(replace-match "false"))
   ((equal (match-string 0) "false")
	(replace-match "true"))
   ((equal (match-string 0) "YES")
	(replace-match "NO"))
   ((equal (match-string 0) "NO")
	(replace-match "YES"))
   ((equal (match-string 0) "nil")
	(replace-match "t"))
   ((equal (match-string 0) "t")
	(replace-match "nil"))
   ))

(global-set-key "\C-c!" 'my-toggle-bool)