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The various listener targets for a particular event class are represented
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level. An event is fired off using code like this::
some_object.dispatch.first_connect(arg1, arg2)
Above, ``some_object.dispatch`` would be an instance of ``_Dispatch`` and
``first_connect`` is typically an instance of ``_ListenerCollection``
if event listeners are present, or ``_EmptyListener`` if none are present.
The attribute mechanics here spend effort trying to ensure listener functions
are available with a minimum of function call overhead, that unnecessary
objects aren't created (i.e. many empty per-instance listener collections),
as well as that everything is garbage collectable when owning references are
lost. Other features such as "propagation" of listener functions across
many ``_Dispatch`` instances, "joining" of multiple ``_Dispatch`` instances,
as well as support for subclass propagation (e.g. events assigned to
``Pool`` vs. ``QueuePool``) are all implemented here.
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