single-part operations
**********************

   Operations to make single-part are following:

`<C-c C-x C-t>'
     Insert single-part tag indicates text part.

`<C-c C-x C-i>'
     Insert file as a MIME attachment.  If `C-u' is followed by it, it
     asks media-type, subtype or encoding even if their default values
     are specified. (cf. See tag specification for inserted file)

`<C-c C-x C-e>'
     Insert external part.

`<C-c C-x C-v>'
     Record audio input until `C-g' is pressed, and insert as a audio
     part. (It requires /dev/audio in default.)

`<C-c C-x C-y>'
     Insert current (mail or news) message. (It is MUA depended.)

`<C-c C-x C-m>'
     Insert mail message. (It is MUA depended.)

`<C-c C-x C-w>, <C-c C-x C-s>'
     Insert signature.

`<C-c C-x C-k>'
     Insert PGP (See PGP) public key. (It requires Mailcrypt
     package.)

`<C-c C-x t>'
     Insert any single-part tag.