(defun delete-selection-mode (arg)
"Toggle Delete Selection mode.
When ON, typed text replaces the selection if the selection is active.
-When OFF, typed text is just inserted at point."
+When OFF, typed text is just inserted at point.
+
+A positive argument turns the mode on, negative argument turns it off,
+and no argument (or nil) toggles the mode."
(interactive "P")
(setq delete-selection-mode
(if (null arg) (not delete-selection-mode)