The model is designed to fire exactly once.
The instance procedure
takes an operation and the time to execute that operation as arguments.
will schedule the event at the appropriate moment.
This routine also redefines
to delete the object instance when the operation is completed.
This notion of the object deleting itself is fragile code.
Since the object only fires once and does nto have to be rescheduled,
it does not overload the procedures up or down.