commenting trouble

[Reynaldo]

Hello,

Recently I wrote this (my first lines of Emacs Lisp)

(defun comentar (&optional beg end)
  (interactive "r")
  (save-excursion
    (if (and beg end)
    (comment-region beg end)
      (let ((beg (progn
           (beginning-of-line)
           (point)))
        (end (progn
           (end-of-line)
           (point))))
    (comment-region beg end)))))

And my purpose was to use the same function to comment a line without
having to select a region, or comment a region (selecting it, of course)
but it doesn't work as expected. If for once I delete the if condition
then it works fine when I comment a line, but i wanted to extend it to
comment a region.

Is this possible? Or does one need to have separate command (and key
bindings) to work on a line or on a region as with kill-line and kill-region

Regards,
Reynaldo