This is a marker interface for receivers that have state
That is, whenever a get method is called on this receiver, the
current token is always available and returned. Further more, if
no put method is called between get methods, the returned tokens are
always the same. Usually, the receivers that implement this interface
have capacity of 1.
Receivers that do not implement this marker interface have event
semantics, which means that when a get method is called on the receiver,
the current token is returned and it is also removed from the receiver.