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A derived class from Listener.
Since receiving an event is characterized by a unique signature of a
method (for example void receives_event1(Event1*)), it is
possible to simplify the definition of the pair (Event,Listener).
The use of the MultiplexerListener allows this feature, by
only defining Event classes, and to listen to these events in
a simple way.
The Subscriber methods to connect to any type of Event
take only the event type as parameter. Using this class, it is
unecessary, when creating a event of a given type, to create its