* gnus-art.el (gnus-mime-recompute-hierarchical-structure): New user option.

* gnus-art.el (gnus-mime-recompute-hierarchical-structure): New user option.
(gnus-mime-multipart-functions): Doc and customization fix.
(gnus-article-mime-hierarchy): New variable.
(gnus-article-mime-hierarchy-next): New variable.
(gnus-article-mode): Make gnus-article-mime-hierarchy buffer-local.
(gnus-article-setup-buffer): Set gnus-article-mime-hierarchy and
 gnus-article-mime-hierarchy-next to nil.
(gnus-insert-mime-button): Show hierarchy numbers.
(gnus-mime-accumulate-hierarchy): New function.
(gnus-mime-enter-multipart): New function.
(gnus-mime-leave-multipart): New function.
(gnus-mime-display-part): Recompute hierarchical MIME structure.
(gnus-mime-display-alternative): Show hierarchy numbers.

* mml.el (mml-preview): Set gnus-article-mime-hierarchy and
 gnus-article-mime-hierarchy-next to nil.