code: protected EventListener addListener(ADDRESST key, EventListener value) {
  		synchronized (mutex()) {
  			return this.listeners.put(key, value);
  		}
  	}
docs: Add a participant with the given address in this repository.
  
  @param key address of the participant.
  @param value is the participant to map to the address.
  @return the participant that was previously associated to the given address.