class type event_system =object..end
The event_system manages events, handlers, resources, groups,
 etc. It is now a class type, and you may invoke the operations directly
 for the class. The operations are still available as functions (below).
A resource is an operation with an optional timer. The operation
 describes the condition to watch for, and the timer defines the
 maximum period of time for that. If the condition becomes true,
 an Input_arrived, Output_readiness, or Out_of_band event
 will be triggered. If the timer expires, a Timeout event will be
 generated. After the event the resource remains active, and the
 timeout period begins anew.
A resource is usually bound to a file descriptor. It is allowed to watch the same descriptor for several different conditions, but it is forbidden to watch the same descriptor for the same kind of condition several times.
As a special case, the operation Wait is not bound to a
 file descriptor, but simply starts a timer. The argument of Wait
 can be used to distinguish between several timers that are active
 at the same time.
Event handlers get the events one after the other, and 
 process them. When a handler is called for an event, there are
 several possible reactions: (1) The handler can return normally,
 which means that the event has been accepted, and will not be
 passed to any other handler. (2) The handler can raise
 Equeue.Reject, which means that the handler cannot process
 the event, and that another handler should get it. (3) The handler
 can raise Equeue.Terminate which means that the event has been
 accepted, and that the handler is terminated (it will never be
 called again). (4) The handler can raise Abort which means that
 the event is deferred, and that a special abort mechanism is
 triggered (see the description for Abort above), this is also
 terminates the handler. The deferred event will again be processed
 in the future. (5) The handler can raise any other exception.
 This causes that the event is deferred, and the exception falls
 through to the caller of run.
Groups are used to simplify the association of events to
 handlers, and to simplify the termination of handlers (see clear).
 If an event is associated with a group, only handlers associated with
 the same group will get them.
There is a special Close handler which is useful to close file descriptors no longer needed. It is called when all resources are removed from the event system dealing with the file descriptor. The close handler should close the descriptor. Note that close handlers are only useful under certain circumstances.
method new_group : unit -> group
method new_wait_id : unit -> wait_id
method exists_resource : operation -> bool
method add_resource : group -> operation * float -> unit
method add_weak_resource : group -> operation * float -> unit
method add_close_action : group -> Unix.file_descr * (Unix.file_descr -> unit) -> unit
method add_abort_action : group -> (group -> exn -> unit) -> unit
method remove_resource : group -> operation -> unit
method add_handler : group ->
       (event_system ->
        event Equeue.t -> event -> unit) ->
       unit
method add_event : event -> unit
method clear : group -> unit
method run : unit -> unit
method is_running : bool
method when_blocking : (unit -> unit) -> unit