Usage: ON [#|+|-|^|&|@]CHANNEL_SIGNOFF [-|^]"<channel> [<nick>]" [action]
  This is activated whenever someone on a channel you are on
  leaves irc.  Normally, the display would show:
    *** Signoff: <nickname>
  but, of course, you can change this.  If <nick> is your own
  nickname, then the action will be taken when you exit
  from irc.  The parameters are as follows:
    $0  The channel from which the person signed off
    $1  Nickname of person who left irc
    $2- Text of Signoff message.