+(defcustom LilyPond-comment-region "%%$"
+ "*String inserted by \\[LilyPond-comment-region]\
+ at start of each line in region.")
+
+(defun LilyPond-comment-region (beg-region end-region)
+ "Comment/uncomment every line in the region.
+Insert LilyPond-comment-region at the beginning of every line in the region
+or, if already present, remove it."
+ (interactive "*r")
+ (let ((end (make-marker)))
+ (set-marker end end-region)
+ (goto-char beg-region)
+ (beginning-of-line)
+ (if (looking-at (regexp-quote LilyPond-comment-region))
+ (delete-region (point) (match-end 0))
+ (insert LilyPond-comment-region))
+ (while (and (zerop (forward-line 1))
+ (< (point) (marker-position end)))
+ (if (looking-at (regexp-quote LilyPond-comment-region))
+ (delete-region (point) (match-end 0))
+ (insert LilyPond-comment-region)))
+ (set-marker end nil)))