One example of code I can't understand

[Richard Stallman <rms@gnu.org>]

Here's more code from mm-util.el that I don't understand.
I don't know what this function is supposed to do
or how it works.


(defun mm-iso-8859-x-to-15-region (&optional b e)
  (if (fboundp 'char-charset)
      (let (charset item c inconvertible)
	(save-restriction
	  (if e (narrow-to-region b e))
	  (goto-char (point-min))
	  (skip-chars-forward "\0-\177")
	  (while (not (eobp))
	    (cond
	     ((not (setq item (assq (char-charset (setq c (char-after)))
				    mm-iso-8859-x-to-15-table)))
	      (forward-char))
	     ((memq c (cdr (cdr item)))
	      (setq inconvertible t)
	      (forward-char))
	     (t
	      (insert-before-markers (prog1 (+ c (car (cdr item)))
				       (delete-char 1)))))
	    (skip-chars-forward "\0-\177")))
	(not inconvertible))))