Synopsis:
   ison [<nick> [<nick> ...]]

Description:
   This takes a list of nicknames, then returns the subset of that list
   that are currently in use on the network.  If none are online, an
   empty list is returned.  This is the mechanism used by NOTIFY.  If
   no nickname is specified, the user's nickname is used (and is thus
   returned).

See Also:
   notify

Other Notes:
   The 303 numeric is used to return ISON queries.  This numeric is used
   by NOTIFY to determine who has signed on or off.  Caution should be
   exercised when hooking this numeric, as it could very well hamper or
   disable the client's internal NOTIFY system.