docs: public void activated(Pipe pipe)
      {
          //  Move the pipe from passive to eligible state.
          Collections.swap(pipes, pipes.indexOf(pipe), eligible);
          eligible++;
  
          //  If there's no message being sent at the moment, move it to
          //  the active state.
          if (!more) {
              Collections.swap(pipes, eligible - 1, active);
              active++;
          }
      }