Wizards can use /cmds/wiz/_announce.c to specify a list of people whose
entrances to and exits from the mud they wish to be informed of.

int announce_user(object who, int type) in the announce daemon is called
who == the user object being announced (a clone of /std/user)
type == 0-3, depending:
0 == User is entering the game.  
Called by:
  /std/user.c complete_startup()
1 == User is leaving the game.
Called by:
  /std/user.c quit()
  /std/ghost.c
  /cmds/std/_su.c
2 == User is re-entering the game.
Called by:
  /std/user.c restart_heart()
  /std/body.c
3 == User is going net-dead.
Called by:
  /std/user.c net_dead()
  /std/body.c

A user won't be informed
 a) of their own movements
 b) if they have no environment
 c) if the who would not be visible to them

The daemon uses the message type "announce", in anticipation of smart clients.