code: @Override
      public void unsubscribe(String channel, ISubscriber<ID, DATA> subscriber) {
          try {
              Set<ISubscriber<ID, DATA>> subs = subscriptions.get(channel);
              synchronized (subs) {
                  subs.remove(subscriber);
              }
          } catch (ExecutionException e) {
              throw new RuntimeException(e);
          }
      }
docs: {@inheritDoc}