code: public void registerCallback(final S symbol, final BiConsumer<S, T> callback) throws BitfinexClientException {
  		final List<BiConsumer<S, T>> callbackList 
  			= callbacks.computeIfAbsent(symbol, (k) -> new CopyOnWriteArrayList<>());	
  		callbackList.add(callback);
  	}
docs: Register a new callback
  @param symbol
  @param callback
  @throws BitfinexClientException