CODE SNIPPET: toggle char at point.

[Sandip Chitale]

(defun toggle-case (arg)
  "Toggle char after point if it is a uppercase or a lower case char,
else simply insert the char."
  (interactive "*P")     
  (let  ((ch (char-after (point))))
    (cond ((not ch) (self-insert-command (or arg 1)) t)
          ((/= (upcase ch) ch) (upcase-region (point) (1+ (point)))
(forward-char 1) t)
          ((/= (downcase ch) ch) (downcase-region (point) (1+ (point)))
(forward-char 1)t)
          (t (self-insert-command (or arg 1))))))

;;; vi like keybinding
(global-set-key [(~)]                       'toggle-case)