enclosure operation
*******************

   Operations to make enclosure are following:

`<C-c C-x a>'
     Enclose specified region as multipart/alternative.

`<C-c C-x p>'
     Enclose specified region as multipart/parallel.

`<C-c C-x m>'
     Enclose specified region as multipart/mixed.

`<C-c C-x d>'
     Enclose specified region as multipart/digest.

`<C-c C-x s>'
     Digital-sign to specified region. (cf. See PGP)

`<C-c C-x e>'
     Encrypt to specified region. (cf. See PGP)

`<C-c C-x q>'
     avoid to encode tags in specified region.  In other words, tags is
     interpreted as such string.  (In current version, it may be
     incomplete.  Maybe PGP-signature does not work for this enclosure.)