docs: protected boolean add(Object socketOrChannel, final ItemHolder holder)
      {
          if (socketOrChannel == null) {
              Socket socket = holder.socket();
              SelectableChannel ch = holder.item().getRawSocket();
              if (ch == null) {
                  // not a channel
                  assert (socket != null);
                  socketOrChannel = socket;
              }
              else if (socket == null) {
                  // not a socket
                  socketOrChannel = ch;
              }
          }
          assert (socketOrChannel != null);
  
          CompositePollItem aggregate = items.get(socketOrChannel);
          if (aggregate == null) {
              aggregate = new CompositePollItem(socketOrChannel);
              items.put(socketOrChannel, aggregate);
          }
          final boolean rc = aggregate.holders.add(holder);
          if (rc) {
              all.add(aggregate);
          }
          return rc;
      }