decoding more complex MIME messages

[David Edmondson <dme@dme.org>]

[ Gnus 5.10.6 on Emacs 21.3.1 ]

I receive some mail messages[1] that have the following form:

multipart/alternative {
  text/plain
  multipart/related {
    text/html
    image/jpeg
    image/jpeg
    ...
  }
}

My preference is to read the HTML version with inline embedded
images.  Unfortunately the code that searches for the images doesn't
find them, as they are nested too deeply.  As a result, I modified
mm-w3m-cid-retrieve-1:

(defun mm-w3m-cid-retrieve-1 (url handle)
  (dolist (elem handle)
    (when (listp elem)
      (if (equal url (mm-handle-id elem))
	  (progn
	    (mm-insert-part elem)
	    (throw 'found-handle (mm-handle-media-type elem))))
      (if (equal "multipart" (mm-handle-media-supertype elem))
	  (mm-w3m-cid-retrieve-1 url elem)))))

In essence, if one of the sub-parts of the element being examined is a
multipart, we descend into the multipart to search.

Could someone check this and, as appropriate, correct it, improve it
or commit it?  I've previously signed FSF papers for Gnus I believe,
if that's relevant.

Footnotes: 
[1]  From newspipe: http://newspipe.sourceforge.net.

dme.