code: public final void writeAndFlush(Collection<? extends Client> clients) {
          if (clients.isEmpty()) {
              throw new IllegalArgumentException("You must send this packet to at least one client!");
          }
  
          clients.forEach(this::writeAndFlush);
      }
docs: Queues this {@link Packet} to one or more {@link Client}s and calls {@link Client#flush()},
  flushing all previously-queued packets as well.
  
  @param clients A {@link Collection} of {@link Client}s.