SUBJECT  -  replace the SUBJECT field

Usage:  SUBJECT [text]

The SUBJECT command replaces the existing subject field with any other
  text you choose.  The SUBJECT command with no text eliminates the field.

Example:
    Send>display subject
     Subject: Some really wonderful news
    Send>subject Nothing much
    Send>display subject
     Subject: Nothing much

For more help, type "help" and one of these topics:
  display   erase   header-field

