/**
     * Unregisters a previously registered listener.
     *
     * @param listener the listener to unregister
     * @throws NullPointerException if {@code listener} is null
     */
    void unregisterListener(@Nonnull Listener listener);

    /**
     * Returns the list of registered listeners.
     *
     * @return an immutable collection of listeners, never {@code null}
     */
    @Nonnull